Fosi IM4 after 100 hours

Sound Over Specs: A Brief Summary

I have a solid baseline for what quality sound and a wide soundstage should feel like, having spent years with high-end gear like the Sennheiser HD-650 and Hifiman Sundara. However, my appreciation for the Fosi Audio IM4 isn't based on the "tech side" or the spec sheet—it is based solely on the sound they produce. Despite the hearing degradation and tinnitus from 20+ years on an aircraft carrier flight deck, these IEMs deliver a lifelike, "airy" experience that stands out through pure performance. Whether it is the superb left-to-right separation or the fatigue-free clarity provided by the swappable nozzles, my liking for this set comes down to how well they translate music in real-world use.

Reviewer Background: I typically use full-sized, over-the-ear headphones. My primary home rigs include the Sennheiser HD-650 and Hifiman Sundara powered by either a FiiO K7BT or a Monoprice Monolith amp. While 20+ years on an aircraft carrier flight deck has definitely degraded my hearing and left me with tinnitus, I have a solid baseline for what quality sound and wide soundstage should feel like.

1. The Hardware & Setup

• IEM: Fosi Audio IM4 (Open-back design).
• Cable: 4.4mm Balanced (Essential for the power these need).
• Tips: "Max Bass" Medium (A great seal is mandatory for open-backs).

2. The Nozzle Duel: Energy vs. Smoothness

Brass Nozzles (The "Critical" Choice)

• Vocal Clarity: On Fleetwood Mac’s "Dreams," Stevie Nicks’ voice was incredibly clear—breathy and crisp, as if she were standing right there. The brass nozzles emphasize that upper-midrange detail beautifully.
• The Massive Attack Test: I used these for "Angel." The brass nozzles did a great job keeping Horace Andy’s unique, vibrating vocals front-and-center while the heavy bass line thundered underneath. Even when the track gets aggressive, the brass keeps the "clack" of the percussion sharp. It’s a great combo, but for my tinnitus, it’s a "short session" setup because of the intensity.

Aluminum Nozzles (The "Productivity" Choice)

• Taming the Beast: These were the winners for my daily work. They reduced the "brashness" in Kendrick Lamar’s "Humble" and made Radiohead non-fatiguing.
• Soundstage: On "Paranoid Android," the left-to-right separation was superb—honestly better than my dedicated test tracks. The open-back design means the sound isn't "trapped" in your head; it feels airy and wide.

3. The Power Problem: Pixel 10 XL Pro

If you’re planning to run these straight off a phone, be careful. My Pixel 10 XL Pro did not have enough juice to drive these to "workout levels" using a standard USB-C cable ($9 Amazon special). It felt thin and weak.

• The Solution: Use a dedicated source. I moved to the FiiO JM21 and the difference was night and day.
• Gain Matters: On the JM21, Medium Gain was perfect. I tried High Gain, but it caused the music to "clip" and distort. These IEMs are sensitive; they want quality power, not just raw volume.

4. Technical Notes: The "Skips"

I originally had some "skips" in the audio. After testing, I’ve realized this was a broadband deficiency (Bluetooth/streaming bandwidth issues on my phone) and not the IEMs themselves. On the FiiO JM21 in "Pure Music Mode," the audio was rock solid.

5. Cables & Accessories: A Quality-First Approach

While the IM4 comes stock with a high-purity, silver-plated 3.5mm cable, I also tested it with the 4.4mm Balanced and USB-C cable variants. Across all three, the build quality is outstanding:

• Tactile Feel: The cables are remarkably soft and pliable. They don’t "fight" you or retain memory kinks, which is a rare find at this price point.
• Microphonics: There is zero cable noise (microphonics). You can move around freely without hearing that annoying "thumping" or "rustling" sound in your ears.
• The Case: The included faux-leather carrying case is much roomier than it looks. It easily houses the IEMs along with two different cables—and at one point, I even managed to fit three cables inside comfortably. It’s a great piece of gear for someone who likes to keep their balanced and standard connections in one place.

6. Source Comparisons: Old School vs. New School

Beyond my modern gear, I also tested the IM4 with my FiiO X3 using the 3.5mm connection. Even on this older hardware, the IM4 played beautifully. It proves that while the Pixel 10 XL Pro struggled for power, a dedicated player—even an older one—is all it takes to make these Beryllium drivers sing. Whether I was using the modern JM21 or the classic X3, the consistency of the "airy" open-back sound remained a standout feature.

Final Verdict: The Pros and Cons

Final Summary: A Reality Check

I’ll be honest: I struggled to find legitimate 'cons' for the Fosi Audio IM4. While my Pixel 10 XL Pro initially struggled for power, I’m now convinced that was either a broadband deficiency or the result of using a cheap $9 cable rather than a fault of the IEMs. Once I switched to a proper source like the FiiO JM21 or X3, they performed beautifully.

Coming from the HD-650 and Sundara, I was particularly impressed by the noise floor. There is absolutely no hiss or hum, even when cranking the volume during silent passages. While high-gain settings on my desktop amps caused some clipping, dropping to Medium Gain offered much better headroom and a cleaner signal.

As for the open-back design, it’s a non-issue for those around you. Unless you’re playing at 'unsafe' levels, the leakage is subtle and shouldn't be annoying to anyone nearby. For a guy used to high-end open-back cans, these IEMs offer a similar 'airy' experience in a portable, fatigue-free package. I'm not trying to 'sell' these, but I am definitely working them into my everyday rotation for 'on the go' music—whether I'm in the yard or the shop—when using high-dollar cans isn't ideal or practical.