Fosi IM4 Review

Disclaimer

Hello everyone! Before we start, I want to let you know that Fosi sent me the IM4 so I could test it, spend some time listening, and share my honest thoughts. I’m not getting paid for this review, and Fosi hasn’t told me what to say, checked my review before posting, or given me any deadlines.

Everything I share here comes purely from my own experience with the IM4. I always try to stay as fair and objective as possible, but at the end of the day, this is still just my personal opinion, and we all have different preferences and ways to judge the products, which is exactly what makes this hobby so enjoyable.

Big thanks to Fosi for giving me the chance to try out the IM4, and of course, a huge thank you to you for reading and supporting my reviews. Your trust really means a lot to me!

Introduction

I have been following and loving Fosi mainly for their small DACs/AMPs and, of course, their i5 Planar headphones. Seeing them step into the IEM world with the IM4 immediately got me excited. Instead of trying to impress with some crazy multi driver flagship and a big price tag, they went for a much slower, easygoing, and sensible first move. A simple single dynamic driver IEM at 99$.

To me that already says a lot about what Fosi is trying to do here. The IM4 feels like their “soft entry” into IEMs, with something that is easy to understand, easy to drive, and just something more simple, rather than a showpiece. At the same time, it is not that simple and basic. Fosi has made this one with an open back, which is pretty unique for an IEM, and they are also aiming at that very crowded segment, where there are a lot of strong competitors and people are very picky about value.

So today, I want to share my experience with the IM4, talk about how it sounds, how it fits and feels, and most importantly, if Fosi’s first IEM can actually stand its ground against the competition in this price range or if it still feels like a “first attempt” that needs more time to mature.

Unboxing

When I opened the IM4 box, it actually felt pretty nice for a 99 dollar set. You slide off the outer sleeve, and inside there is this textured black inner box with the Fosi logo. Lifting the lid, you see the two shells sitting in a foam with the carrying case underneath, and inside you get the cable, the tips, and the extra nozzles.

The case is one of the first things that stood out to me. It looks good, feels good, and I can easily fit the IEMs, the cable, and the tips inside without fighting it. For daily use, that is a very convenient and good looking case.

The stock cable is mostly lovely too. It is a light, braided, not too thick and not too thin. I personally prefer thicker cables, that is just my taste, but I still quite like this one. My only real complaint is that it comes only with a 3.5 mm plug, while today many IEMs at this price already include modular cables or at least a 4.4 option.

For tips you get three types of silicone sets: Balanced, Bass, and Deep Bass. The main difference is the bore size, with Balanced being the widest and Deep Bass the narrowest. I am not a fan of this particular tip design because of the molding line in the middle, which makes my ears feel itchy and a bit scratchy, but it is still nice to see three different tuning options included in the box.

Technical specifications

Specs

• Driver: 10 mm dynamic driver, N52 dual magnet, dual cavity motor
• Diaphragm: PU diaphragm with beryllium coating
• Acoustic design: dual chamber, open back housing
• Shell material: CNC machined 6063 aluminum alloy, sandblasted and anodized
• Impedance: 21 Ω
• Sensitivity: 109 dB
• Frequency response: 20 Hz to 20 kHz
• Cable: 4 core, 392 strand 5N OFC silver plated copper, detachable 0.78 mm 2 pin
• Plug: 3.5 mm gold plated plug included, 4.4 mm cable available separately
• Swappable nozzles: aluminum and brass, for smoother or slightly brighter tuning options

Shell design and comfort

The IM4 is sold as an open back IEM, but in reality I would call it semi open. There are visible vents and that front grill, which I can clearly see through, and the nozzle, so it does not isolate like a normal closed IEM. I would guess that it’s closer to the Kiwi Ears Septet and FiiO FH19 in its openness rather than to something like the Audeze open back IEMs. It cuts some outside noise, but if someone talks near me, I still hear them. For home, office, or quieter places, it works fine, but it is not going to be a good pick for a loud bus or train.

Where this design really shines is comfort. Because the shell is so well vented, I do not get that pressure buildup or “plugged ear” feeling at all. I can sit with the IM4 in my ears for hours and hours and forget that it is there. The aluminum shells are light, shaped nicely, and for me they just disappear during use.

On top of that, you get swappable nozzles. There is an aluminum one and a brass one in the box. The brass nozzle sounds a touch brighter to my ears, while the aluminum one feels a bit smoother. I will talk more about how they change the sound later in the review, but it is cool to have this kind of simple tuning option included.

Driver and inner tech

Inside, the IM4 is quite simple compared to multidriver IEMs, with a single 10 mm dynamic driver. The diaphragm is PU with a beryllium coating, driven by an N52 dual magnet system and a dual cavity driver. The idea is to get that classic single DD natural sound signature, with theoretically a bit more control and speed from the diaphragm.

Fosi also uses a dual chamber acoustic design. The front and rear chambers are tuned with ports and damping to keep the response balanced and to avoid harshness. On paper it sounds like a modern, carefully executed single DD setup rather than just a random driver thrown into a shell, and that actually matches what I am hearing in practice.
Weight: about 7 g per earbud, around 38 g including the cable.

Sound performance

Bass

To my ears the IM4 has a pretty balanced overall tuning. The bass hits with good punch and presence, so kick drums and bass guitars have weight, but the sub bass feels a bit held back, and it doesn’t go as deep and rumbly as I would love it to. On tracks that really dig into the lowest notes, I sometimes miss that last bit of rumble and depth. With busier songs that ask for fast, tight bass, things can get a little messy, to be honest. For example, with Everybody Walkin’ This Land by Home Free, the bass and lower mids can blur together, and the different voices sometimes sound a bit smeared instead of cleanly separated. That’s the biggest complaint that I have regarding the bass, but still, it’s a single DD, and I can’t expect from it the same performance as from a good multidriver.

Mids

The mids follow the same pattern. There is body and warmth, but that extra mid bass energy bleeds a bit into the lower mids and makes them feel slightly dirty and muddy at times. It is not a disaster, but it is noticeable if you listen for separation and clarity. The upper mids, though, are actually pretty nice. Female vocals come through well, and as usual I tested with Patty Gurdy. When she climbs into the higher notes, the IM4 keeps her voice clean and smooth without sounding shouty or sharp, which I really appreciate.

Highs

The treble is where the IM4 plays things quite safe. The highs sound good and never harsh to me, but they do not reach as far or as sparkly as I personally like. There is a bit of early roll off up top, so you lose some of that ultra fine air, micro detail, and long lasting decay. The upside is that this set is very easy on the ears. Nothing in the treble pokes out or becomes fatiguing, and combined with the open design that relieves pressure, it turns the IM4 into a very comfortable IEM for very long listening sessions, or like I have been using it, all day long.

Technicalities

On the technical side, I would say the IM4 is solid for a single DD at this price. The soundstage is quite big for an in ear. Placement of instruments and vocals is generally good, but when the music gets dense, that mid bass can make things feel a bit congested. Detail retrieval is decent, but it does not reach the level of a well tuned multi driver set with a good crossover, and again, that relaxed, rolled off top end holds back some micro detail. Still, for a single DD at 99$, the technicalities are quite nice and in line with what I expect.

Nozzles and tips

Fosi gives you two sets of nozzles, one aluminum and one brass. The brass nozzles make the upper mids and lower treble a bit more forward. It adds some extra presence to that region, but for me it pushes things a bit too much and makes the tuning feel less balanced up top. With the aluminum nozzles, the sound feels more even and natural to my ears, so I ended up preferring the aluminum nozzles and sticking with them.

As for the tips, you get three types labeled Balanced, Bass, and Deep Bass. On paper they should change the sound because of the bore size, but in my case the actual tonal difference was not very big. It might be related to my ear canal shape and how they seal. I mostly chose the Bass tips simply because I liked their structure and texture the most, and they gave me the most comfortable fit.

Overall I would call the IM4 a safe tuned IEM. Nothing is overly sharp or aggressive, there is a bit too much mid bass and lower mids, which can make things muddy sometimes, but there is no piercing treble and no crazy peaks. Together with the open design and zero pressure buildup, the IM4 ends up being a very easy IEM to live with for long days of listening, even if it is not the most clinical or detailed set out there.

Comparison

I chose to put the IM4 next to some other sets I know well, like the Letshuoer DX1 because it’s a higher priced single DD IEM, the Simgot EA500 because it’s a cheaper single DD IEM, and the Myer Audio SLIIVO SLA3 because it’s similarly priced, but this time a multidriver. This way I think it starts to make a bit more sense where it sits in the market.

Simgot EA500 (90$)

The EA500 is clearly the brightest one of the group. It is not painfully bright or anything like that, but it definitely has more upper energy and more detail than the IM4. At the same time, the bass on the EA500 is weaker and less defined, so to me it feels less balanced overall. The upside is that its mids and mid bass stay cleaner and less muddy than on the IM4.

Letshuoer DX1 (160$)

The DX1, on the other hand, is the leanest and most neutral sounding of the four. It has a very even, well balanced tuning that I like a lot, but since it is also a single dynamic driver, I sometimes feel it struggles when the music gets very busy and complex, a bit like the IM4 but in a different way. Where the IM4 leans into warmth and mid bass, the DX1 stays thinner and more controlled, which can sound a bit cleaner but also less engaging if you enjoy some extra weight. In terms of physical build, the DX1 is actually the smallest shell of them all, yet it feels thick and heavy, and it is also the most expensive set in this group.

Myer Audio SLIIVO SLA3 (104$)

The SLA3 is on a different level in terms of tuning. For me it is the most well tuned of all four, and that is not a surprise considering its multi driver configuration. It has strong bass and sub bass, but it manages to keep the mids clean and well separated, without the mud that the IM4 has. Everything feels more organized and better layered, with better separation and more control. On top of that, I think the SLA3 has the best overall package, with great eartips and an excellent modular cable that really adds to the experience right away.

Even with that, I still find myself reaching for the IM4 quite often for one simple reason, which is comfort. Thanks to its open back design and the way it deals with pressure, I can sit with the IM4 in my ears for very long sessions without any discomfort, and that is something the others do not match in the same way. The SLA3 comes second for me in comfort, with a very ergonomic, lightweight shell, but the IM4 wins when I just want to wear an IEM all day and forget it is there, even if it is not the cleanest or most technically impressive set of the bunch.

Conclusion

After testing the IM4 for a while, I feel like I can sum it up pretty simply. This is not a crazy technical, ultra detailed IEM. It is a safe, easy going set with a warm, strong mid bass, relaxed treble, and a focus on comfort and very long listening.

The downsides are clear. The mid bass can bleed into the lower mids and make things a bit muddy, the sub bass could have more depth, and the treble does not have the extension and air that I like to have. If you are chasing maximum detail and separation, there are cleaner and more precise options out there.

But on the other hand, the IM4 is genuinely comfortable, thanks to the open back design and the low pressure buildup, and it is very easy on the ears tonally. I can wear it for hours without any fatigue, and that is something I really appreciate. Compared to sets like the EA500, DX1, and especially the SLA3, it is not the best tuned or the most technical, but it is one of the most relaxed and easy to wear all day long.

So for me, the IM4 makes sense for someone who is sensitive to treble, likes a warmer and fuller presentation, and wants a first IEM that is simple, open back, and comfortable for all day use. As a first try from Fosi in the IEM world, it is not perfect, but it is a solid and very promising start, which makes me very curious about what they will come up with next.

Pros

• Very comfortable open back design with no pressure buildup, which is great for long sessions
• Warm, easy going tuning that is non fatiguing and treble safe
• Big and open sounding stage for a single dynamic at this price
• Swappable aluminum and brass nozzles
• Nice looking, practical carrying case that fits all the accessories
• The stock cable feels quite nice in the hand

Cons

• Mid bass bleed makes the lower mids a bit muddy on busy tracks
• Sub bass lacks some depth and rumble
• Treble rolls off early, so detail and air are not the best
• Isolation is weaker than regular IEMs because of the open back design
• Only includes a fixed 3.5 mm cable, no modular or 4.4 option

Where to buy:(Non-affiliated! None of my reviews use affiliated links!)

Fosi Official Store