In this article, I review the Fosi Audio IM4 IEMs. Fosi Audio is known primarily for its DACs and Amplifiers but they have recently started producing products for the portable market, including the IM4 and a series of dongle DACs. The IM4 features an open-back design and a 10mm PU Beryllium-coated Diaphragm. It’s priced at $99 USD.

Disclaimer: This sample was provided by Fosi Audio for an honest review. All observations and opinions here are my own, based on my experience with the product.

Fosi Audio IM4

Sound

Sources used for testing include the Fosi Audio DS3, iBasso DX180 and D&A Alpha Pro. The Fosi Audio IM4 are very efficient and easy to drive, reaching healthy listening levels straight from a smartphone or small dongle DAC without needing extra amplification.

The IM4 has a warm, light V-shaped sound signature. This review is based on the default black nozzles. Switching to the included gold nozzles adds a bit more zest and treble detail, but at the expense of some extra edginess in the highs.

Bass

While not a basshead IEM, the IM4 delivers punchy bass with an emphasis on the mid and upper bass. The sub-bass is slightly pulled back compared to the mid-bass, but extension is still good and it can produce a light rumble when needed. There’s a little bass bleed into the mids, though overall control remains solid.

Midrange

The midrange is slightly warm and musical, bolstered by the warmth that carries over from the bass. As a result, male vocals sound rich and chesty, while female vocals inherit a touch of sweetness and still manage to soar.

A subtle peak around 5kHz adds some extra clarity and, in games, helps footsteps stand out more clearly from background noise. It also gives transients a sharper attack, but thankfully without pushing things into glare or fatigue.

Treble

The IM4’s treble is mostly smooth and fairly easygoing, avoiding the kind of sharpness that can make brighter IEMs tiring over time. There’s enough energy up top to keep cymbals and high-pitched details audible, but it doesn’t push hard for sparkle or bite. The open-back style shells seem to help here, giving the top end a slightly airy feel rather than sounding closed-in or congested.

On some tracks, there is the occasional metallic edge to notes, particularly when the upper mids and lower treble are more active, but it’s not a constant issue. With the stock black nozzles, the treble comes across as smoother and a bit more relaxed, while the gold nozzles bring out more detail and presence, sometimes at the cost of a slightly sharper tone.

Soundstage & Technicalities

The IM4 presents a soundstage with decent width and depth, though it’s not as expansive as you might expect from an open-back design. It doesn’t feel cramped, but it also doesn’t really stretch out beyond the head in the way some sets can. Detail retrieval is pretty good overall, but finer micro-details sometimes get slightly smoothed over in favour of a more musical, forgiving presentation.

Resolution is solid for the price, and positional cues come through clearly enough for both music and gaming. The main technical drawback is the occasional metallic timbre on cymbals, which can stand out on certain tracks, but aside from that, the IM4 holds together well in terms of staging and imaging.

Verdict

Overall, the Fosi Audio IM4 is a solid debut IEM from the brand. I really like how they’ve managed to give the shells a proper Fosi aesthetic, with a look that somehow feels in line with their amps and DACs rather than a generic off-the-shelf design. Build quality is excellent, and the accessory bundle is well put together, making the package feel complete straight out of the box.

Sound-wise, they’ve done a good job and most listeners should be happy with the tuning, especially if they’re after something warm, musical, and easy to listen to. My only real reservation is the price, which feels a little higher than I’d like given the competition. Still, the IM4 leaves a positive first impression and definitely has me looking forward to seeing what Fosi Audio does next in the IEM space.

Specifications

Shell Material: 6063 CNC Aluminium Alloy
Driver Unit: 10 mm N52 Dual-Magnet Dual-Cavity Dynamic Driver
Diaphragm: PU Beryllium-coated Diaphragm
Sensitivity: 109dB
Impedance: 21Ω
Frequency Response: 20Hz–20kHz
Cable: 4-core 392-strand 5N OFC Silver-plated Copper Cable
Connector: 2-PIN 0.78mm
Audio Jack: 3.5 mm Gold-plated Plug
Weight (per earbud): 7g ±0.5g
Weight (incl. cable): 38g ±1g