Fosi Audio making an IEM caught my attention because I knew the brand for amplifiers. Reddit and Head-Fi impressions were enthusiastic, suggesting that some of its tuning experience had carried across. After trying it, I was more interested in its own engineering choices than whether it followed the latest trend. I wanted to hear how well it justified those recommendations below $100.
Unboxing and Design
The first thing that caught my eye when opening the package was the soft-textured faux leather case. The tactile feel when holding it in actual use is quite pleasant. Most impressive, however, is the ear tip configuration. It comes with a generous total of nine pairs—three pairs each of balanced, bass, and deep bass tips—allowing users to precisely choose based on their ear shape and sound preference. In addition, two types of tuning nozzles (alloy and brass) that alter the sound signature are included, making the accessory lineup feel remarkably complete.
The smooth aluminum-alloy shells are CNC machined, giving them a solid finish without leaving much weight on the ear. The mesh at the rear also tells you something about their construction: these are semi-open rather than fully sealed. In actual use, the light feel and the shape let them sit comfortably, while the ventilation keeps pressure from building up. That makes a difference over longer sessions, when an enclosed feeling can become tiring even if the music itself is enjoyable. For me, the ability to listen without that plugged-up sensation is one of the strongest attractions of the design.
Core Sound Analysis
For testing, I used the SMSL DO400 for desktop, SMSL DS20 for portable use, and the NICEHCK Octive dongle DAC. The earphones were tested with the brass nozzle and bass tips as the baseline.
The highs highlight the delicate nature characteristic of Balanced Armature (BA) drivers, delivering even the minute vibrations of instruments without missing a beat. The semi-open-back structure naturally vents frequencies that might otherwise sound harsh, allowing you to fully enjoy a refreshing, open feeling without any stuffiness. I liked how the clean decay makes cymbal and hi-hat sounds exceptionally transparent.
The midrange is where the unexpected charm of these earphones lies. Despite the U-shaped tuning emphasizing lows and highs, the vocals cut clearly right through the instruments and step forward. Based on the Harman target while enhancing the resolution of the vocal range, the close delivery of even a female vocalist’s breathing is satisfying. I could truly understand why overseas reviewers praised the natural texture of the vocals.
The bass focuses on texture and transient response rather than sheer quantity. The punch delivered by the 10mm dynamic driver is firm yet disciplined, moving without interfering with other frequency ranges. Thanks to the open-back structure, bass reverberation escapes cleanly without lingering muddiness, keeping the overall sound presentation very pristine. For those seeking massive, heavy impacts, the sound might feel a bit refined, but it hits the right spot in terms of balance.
Resolution is top-tier for this price bracket, and the semi-open-back design provides an extraordinary sense of space. The rich airiness and wide soundstage stretching across left and right—much broader than standard closed-back earphones—offered a quite refreshing experience. Thanks to the precise imaging that clearly separates the position of each instrument, I could fully enjoy a three-dimensional, live-venue immersion when listening to live recordings or orchestral pieces.
Material-Specific Tuning Nozzles
I compared the stock alloy and brass nozzles of the IM4 alongside four compatible NICEHCK brass and stainless steel (SUS) nozzles.
- Alloy Nozzle = Standard for All-Rounders
The fitted alloy nozzle is regarded in overseas discussions as the safest all-round choice. I hear why: the balance is smooth, with little abrasive treble and no jarring peaks. It emphasizes the comfortable side of the Harman target and is easy to keep listening to.
- Brass Nozzle = A Touch of Airiness and Brilliance
The brass option brings more energy into the upper range, making the presentation brighter and more open. I found the added air and the livelier vocal presence appealing, with a more dimensional impression to the sound. That benefit has a qualification: on some tracks the stronger upper mids can also feel a little piercing. This is the choice I would explore for clearer, more brilliant treble and vocals, provided that extra energy fits my tolerance. It offers a different emphasis rather than an automatic improvement for every song.
- SUS (Stainless Steel) Nozzle = Tight Density and Quiet Background
This nozzle tightens the density of the sound. Unwanted resonance in the bass is controlled, and the overall background of the sound becomes noticeably quieter and more composed. With sound decays ending precisely without bleeding, it is ideal when you want to hear drum impacts or basslines with greater clarity.
- Titanium Nozzle = Micro-Details and Ultra-High Extension
This is a tool that maximizes the hi-fi potential of the IM4! While the notes become slightly thinner, the extension in the ultra-high frequencies increases dramatically, capturing subtle reverberations that are easy to miss on standard nozzles. It delivers a cool yet clear sound signature suited for modern pop music.
Pros and Cons
I strongly recommend this to:
Those who value an open soundstage, wide spatial imaging, clear vocals, pressure-free comfort, and the fun of nozzle customization.
I hesitate to recommend this to:
Commuters on public transit, fans of massive sub-bass punch, and users in quiet public places.
Conclusion…
Using the Fosi Audio IM4 made me realize that amplifier-tuning expertise translates significantly to sound tuning as well. The attempt to combine the advantages of an open-back design while retaining the resolution expected of an in-ear monitor feels quite successful. However, because of the semi-open-back structure, you do need to be mindful of your volume in quiet environments, and external noise leakage is something to keep in mind. Even so, the vocal clarity and openness elevated the quality of everyday music listening to the next level. Sometimes, when stepping away from tightly sealed closed-backs to feel the air and sound across a wide stage, this will likely be the first earphone I reach for.
For an entry-level IEM, it makes a convincing case.
