The Fosi Audio IM4 feels like the kind of first serious IEM attempt that does not want to act small. Fosi Audio is better known for compact DACs, amplifiers, and desktop audio gear, so an in-ear monitor from the brand immediately carries a bit of curiosity. The interesting part is that the IM4 does not come across as a timid side project. It aims straight at the crowded $100 segment with a metal shell, detachable 2-pin cable, interchangeable tuning options, a generous tip selection, and a sound that leans lively rather than overly safe.
Fosi Audio IM4
What stands out most to me is the balance between value and ambition. This is a single dynamic driver IEM, using a 10mm beryllium-coated driver, and the overall direction is energetic, forward, and accessible. It is not trying to be a soft background listen. It has punch, presence, and enough treble energy to make the presentation feel clean and immediate.
At the same time, the IM4 is not without caveats. The upper mids and treble can divide listeners, and how comfortable its brighter side will be over long sessions. Still, as a complete package, it feels surprisingly mature for a brand stepping into this category.
Unboxing and First Impressions
The unboxing experience is better than I would expect from a first-generation IEM in this price range. The IM4 arrives as a complete kit rather than a barebones product, and that helps it make a strong first impression. The packaging includes the earphones, a 3.5mm cable with microphone, a basic but useful carrying case, and three types of eartips: Balanced, Bass, and Deep Bass.
I like that Fosi gives the user some tuning flexibility right away. The Balanced tips use a wider bore, while the Bass and Deep Bass tips are meant to shift the low-end character. The differences are not just decorative. The included tips give the IM4 a more customizable feel, which matters because this earphone already sits in a tuning zone where small changes in fit and tip choice can affect the final balance.
The carrying case is simple, but it does the job. I would not call it luxurious, but I would rather have a practical case and a useful spread of tips than flashy packaging with fewer functional accessories. For the price, the IM4 feels like a thoughtful kit.
Fosi Audio IM4 Accessories
Build Quality and Design
The IM4 makes one of its strongest arguments before the music even starts. The shell uses 6063 CNC aluminum alloy, and the design has a clean, minimalist look that suits Fosi Audio’s broader hardware identity. It is available in black Obsidian and silver Galaxy finishes, both of which fit the restrained aesthetic. I prefer this kind of design to overly busy faceplates, especially when the machining and shape already give the IEM enough visual identity.
Despite the metal construction, the earpieces are lightweight, with each side listed at 6.4g without eartips. That matters because metal shells can sometimes feel dense or intrusive in the ear. Here, the IM4 appears to keep the premium feel without turning comfort into a tradeoff. The fit is also helped by the stock eartip selection, and the sizing feels standard enough that most listeners should be able to start with their usual size.
The detachable 2-pin 0.78mm connection is another important point. At this price, I always prefer a standard detachable cable because it extends the lifespan of the product and makes replacement or cable rolling much easier. The included cable is a 3.5mm single-ended cable made from 5N OFC silver-plated copper, and it feels like a good match for the package.
The open-back style design is more interesting visually than transformative sonically. I would not buy the IM4 expecting a headphone-like open-back effect. It may help with pressure relief and airflow, but the practical downside is sound leakage. That means it is not the most discreet option for very quiet shared spaces.
Sound Quality
The IM4 has a lively, forward sound with a V-shaped tilt and a strong sense of clarity. To my ears, its personality is built around punchy bass, present upper mids, crisp treble, and a soundstage that feels respectably wide for an IEM. It is not a dark or relaxed set. It wants to sound immediate and engaging.
Bass
The bass is moderately elevated, with more emphasis on mid-bass punch than deep sub-bass rumble. That gives kick drums and bass guitars good body, and it helps the IM4 sound energetic without becoming slow or muddy. I would not call it a basshead IEM in the pure sense, especially if the goal is maximum sub-bass pressure, but the bass does not feel weak.
The included tips matter here. With the Bass or Deep Bass tips, the low end can become fuller and more organic, while the Balanced tips keep the presentation cleaner and more open. I like this approach because it lets the IM4 cover more listening preferences without relying on electronic EQ.
Midrange
The midrange is clear, forward, and expressive. Vocals do not feel buried behind the bass, and instruments have good definition. The IM4 can sound especially engaging with pop, rock, acoustic material, and modern music where vocal presence matters.
The main caution is the upper-midrange energy. This part of the tuning gives the IM4 its sense of clarity and immediacy, but it can also become intense depending on the recording and volume. Opinions differ here. Some listeners will enjoy the extra presence and detail, while others may feel the IM4 pushes too hard in the upper mids.
Treble
The treble is clean, extended, and fairly crisp. Cymbals and hi-hats have good definition, and the top end contributes to the IM4’s sense of resolution. I do not hear it as a dull or overly smoothed IEM. It has bite, sparkle, and enough air to keep the sound from feeling closed in.
That said, treble sensitivity will be the deciding factor for some buyers. Some listeners may find the treble controlled and not harsh, while others may find it too much during longer sessions or brighter tracks. I would be careful recommending it to anyone who already avoids energetic upper mids and treble.
Technical Performance
For the price, the IM4 performs well technically. Imaging is good, separation is clean, and the soundstage expands more laterally than front-to-back. It gives enough space for gaming and busy arrangements, and it avoids sounding congested most of the time.
I would not call it class-leading in every technical area, but it is solid. Resolution is good, the presentation is organized, and the dynamic driver keeps the sound coherent. The IM4’s technical strength is not just raw detail; it is the way it combines clarity, punch, and a reasonably spacious stage in a simple single-driver setup.
How It Compares
The most useful comparison is with the Sennheiser IE 200, because both sit in the single dynamic driver conversation, though at different price points. The IM4 comes across as the more energetic and full-bodied option, with stronger presence, more punch, and a more immediate sound. The IE 200 still has an advantage in airiness and a more relaxed sense of refinement, but the IM4 feels like the better value if you want a livelier, more complete package for less money.
Against very budget options like the GK Kunten, the IM4 feels more mature in build, accessories, and overall presentation. The GK Kunten can deliver more sub-bass emphasis and a more instantly fun low-end atmosphere, but the IM4 counters with better construction, a more premium cable system, and a more balanced overall package.
Compared with technically strong competitors around this bracket, the IM4 does not automatically dominate. Sets like the Simgot EA500 LM can still feel sharper in technical execution depending on what you value. The IM4’s strength is that it does many things well at once: build, comfort, accessories, punch, clarity, imaging, and everyday usability.
Is It Right for You?
The Fosi Audio IM4 is for listeners who want an energetic $100 IEM with strong build quality and a complete accessory kit. I would put it high on the list for someone buying a first serious IEM and wanting something that feels more premium than the usual entry-level plastic shell.
It works well for pop, rock, electronic music, metal, acoustic tracks, and gaming. The bass has enough punch to keep music lively, the mids bring vocals forward, and the imaging is good enough to make positional cues in games feel clear. It is also easy to drive, so it does not need a powerful source to sound engaging.
I would be more cautious if you are treble-sensitive or prefer a warm, relaxed, dark presentation. The IM4 is not that kind of earphone. It is brighter, more forward, and more energetic. If that sounds exciting rather than risky, it could be a very good match.
Final Verdict
The Fosi Audio IM4 is a surprisingly confident entry into the IEM market. It combines a clean aluminum design, detachable 2-pin cable, good accessories, useful eartip options, and a lively sound that feels competitive at $99.
Where to Buy
You can find the Fosi Audio IM4 on:
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