Fosi Audio IM4

Lively, spacious sound at a very friendly price

Fosi Audio’s IM4 lands in the increasingly competitive budget IEM space with something unusual: an open‑back shell giving I feel a much wider open soundstage than any other IEMs in this price range. But the rest of the tuning is just as interesting, so read on to find out.

Fosi Audio are a company that I have admired for many years, some of my first reviews were of their high quality class D amplifiers. If you’re curious, see my older reviews of Amplifiers: V3 and ZA3

But in 2025 they have moved well beyond these class‑D roots and impressed in many new categories, including open‑back
i5 planar and even the Luna3 Turnable

Taken together, they show a company unafraid to move into new audio territories and immediately compete.

So after enjoying the IM4 for a few weeks now I feel the default tonality isn’t exactly my personal preference, but it isn’t going for a ‘popular’ tuning like Harman or JM-1 as I feel Fosi wanted to carve a different niche, and are targeting providing that impressive soundstage over tonal accuracy.

So does the IM4 compete in a highly competitive sub $100 IEM market?

Let’s find out in this review:

Unboxing and Build Quality

The IM4 arrives in a tidy package with a neat box:

The back of the box has the key specifications:

Removing the outer sleeve reveals a textured black inner box with the Fosi logo, a nice touch at this price:

Lifting the lid, the two shells sit in a foam tray with the carrying case tucked underneath. Inside you’ll find the detachable cable, three sets of silicone tips, the two extra nozzles, and the usual paperwork.

The included carry case and a broad ear tip selection to fine‑tune fit and sound with specially targeted tips for adjust the bass and midrange:

The eartip selection is very good:

Overall, the accessory pack is high quality and very generous for the price with the extra ‘gold’ nozzle in some protective plastic:

Build overview

The high quality shells are made from CNC‑machined 6063 aluminum alloy, sandblasted and anodized, with an orange metal grille and logo that add some visual pop. The open‑back vents help contribute to the more spacious presentation. Everything feels solid, and the machining around the nozzle threads is clean. The cable is better than typical budget fare: soft, not microphonic, and it sits nicely over the ear.

So the nozzle is swappable between the stock one seen below and the gold one in the box:

The 2pin cable end is nice with clear blue and red indicators:

The 2‑pin 0.78 mm connection and 3.5 mm plug are standard:

But I believe you can order a 4.4mm option from the Fosi Audio Store.

Fit and Comfort

Comfort is excellent. Each earpiece has a satisfying but not heavy mass, the open back design means there is never a build up of pressure, and the contour of the shell sits naturally in my ear:

I could wear the IM4 for multi‑hour sessions issues as it has a nice opinionated shape:

But the ear tip selection matters a bit for final tonality and seal; the included assortment covers a range from narrower to wider bores:

If you struggle with deeper nozzles, the IM4’s moderate insertion depth should be friendly to most ear shapes.

There is even a quick ‘wearing guide’ is included and is helpful if you’re new to over‑ear cable routing.

Features and Performance

Swappable nozzles and tips

The IM4 includes two nozzle materials — Standard and Brass/Gold — that subtly nudge the tonality. In practice, the Brass nozzle sounds a touch brighter to my ears, while the Aluminum one feels a bit smoother. The supplied eartips further refine bass presence and treble openness; between nozzles and tips, you can dial the IM4 toward a slightly warmer or slightly brighter balance without breaking the core tuning.

Open‑back design

Unlike typical IEMs, the IM4’s open‑back construction trades isolation for a more natural, airy presentation. In reality it behaves more like a semi‑open IEM: there are visible vents and a front grille you can see through, so it doesn’t isolate like a closed design. It cuts some outside noise, but if someone talks nearby you’ll still hear them. Great for home, office, or quieter spaces; not ideal for a loud bus or train. The upside is a sense of space that feels less closed‑in than most IEMs around this price.

I feel Fosi Audio have done a clever job with both the tuning and this open back design as both work together to give a much more spacious and lively soundstage than you should expect at this price point.

Driver and inner tech

Inside, the IM4 is quite simple compared to multi‑driver 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 motor. Fosi also uses a dual‑chamber acoustic design with tuned ports and damping to keep the response balanced and avoid harshness. On paper this is a modern, carefully executed single‑DD setup — and it lines up with what I’m hearing in practice. Weight is about 7 g per earbud and roughly 38 g including the cable.

Sound Impressions

Bass

The IM4 has a tasteful low‑end lift that brings warmth and punch without sliding into muddiness. Sub‑bass reaches down confidently, while mid‑bass is rounded and a touch elevated, giving kick drums satisfying weight and texture instead of one‑note thumps. A few go‑to checks: Billie Eilish – bad guy shows firm, well‑timed thuds that stop cleanly; Massive Attack – Angel delivers a steady, ominous rumble without blooming into the mids; and Hans Zimmer – Why So Serious? (0:03–0:30) highlights the sub‑bass sweep where the IM4 stays composed and free of rattle. On Daft Punk – Get Lucky, kick definition comes through cleanly and never masks the rhythm guitar. If you’re a pure bass‑head you may want more quantity, but for balance, speed, and texture at this price the IM4’s bass is both fun and controlled.

Midrange

Lower mids are slightly relaxed relative to bass/treble in keeping with a lively V‑shape, yet vocals retain clarity and a natural timbre. Male vocals like Gregory Porter – Hey Laura sound steady and uncoloured; Adele – Hello and Norah Jones – Don’t Know Why have a present, breathy intimacy without shout. Acoustic instruments are convincing: on Fleetwood Mac – Dreams the guitars have realistic body, and Damien Rice – The Blower’s Daughter shows the IM4’s ability to keep string overtones smooth. Piano and strings on Max Richter – On The Nature Of Daylight are rendered with a pleasing, slightly romantic warmth rather than clinical coolness. If you prefer a mid‑forward studio monitor tonality, you may want a more neutral set, but the IM4’s midrange lands as musical and non‑fatiguing.

Treble

Treble is clean and reasonably extended, adding just enough sparkle to illuminate detail without tipping into harshness for me. Hi‑hats and ride cymbals on Steely Dan – Aja are articulate and crisp; the ride in Miles Davis – So What has the right metallic sheen without excessive bite. String harmonics in Vivaldi – The Four Seasons (any modern DG/Archiv take) have air and separation, and sibilance checks like Sia – Chandelier remain controlled. Tip rolling can fine‑tune the top end: wider‑bore tips open the air; narrower‑bore tips smooth things slightly. Swapping to the Brass nozzle adds a touch more brilliance for treble lovers, while the Aluminum nozzle is the safer, smoother choice.

Soundstage & Imaging

This is where the IM4 really punches above its class. For an IEM, the stage feels notably open, with width that extends past the ear and a convincing sense of depth and height. The center image locks in solidly, and separation between instruments is clean even in busy mixes. On Eagles – Hotel California (Hell Freezes Over), audience ambience wraps around with a gentle, airy halo while guitars and congas occupy distinct, stable positions. Yosi Horikawa – Bubbles is a joy: the droplets arc convincingly across the field with precise lateral motion and front‑to‑back cues. Binaural recordings like Amber Rubarth – Tundra (Chesky) showcase head‑turning positional accuracy; the semi‑open design helps the IM4 sound less “in‑the‑head” than most closed IEMs in this bracket. It’s not over‑ear headphone scale, of course, but within the sub‑$150 IEM space the IM4’s spaciousness and imaging confidence are standout strengths.

Overall, this is a fun, engaging V‑shape that works across pop, EDM, rock, and casual listening. If you prefer a dead‑neutral, mid‑forward tuning you might want something different; if you want toe‑tapping energy with controlled bass, crisp detail, and a delightfully open stage for the money, the IM4 delivers.

Specifications and Measurements

Measurement

Note: All these and 100’s of other measurements are available on my IEM measurement database here

Let’s start with the frequency response:

Overall, as you can see I got an excellent channel balance, and I have highlighted a few areas of interest in this graph:

1. Nice bass extension but not for bass heads
2. A little recessed midrange
3. A midrange forward tuning in this area will bring out the vocals and accoustic intruments in tracks
4. Treble is nice and safe but still interesting.

These 4 tweaks are what gives the IM4 its interesting tuning and definitely help it get a very nice soundstage with a clean forwarding sounding vocal / presense region.

The Gold Nozzle increases the treble so I would not recommend it but I guess it is a good option for those who wanted a more technical sound:

This tuning won’t be for everyone, but I feel it is a well executed and interesting tuning especially for those who want more than just a ‘Harman’ or a ‘New Meta’ tuning.

I thought it was similar enough to the Hidizs Mk12 that I recently review:

With slightly better bass and treble than the well regarded Hidizs Mk12.

Comparing to one of my favourite midrange forward IEM of the last few years the Kiwi Ears Orchestra Lite and that IEM now has a sequel (Orchestra II) that is also midrange forward:

The IM4 has an excellent bass (1), but that recessed midrange (2) does stand out. I think that midrange with the slightly forward presense (3) gives that IM4 that unique soundstage feel that is one of the best I have heard. The treble (4) is a little safer than the 2 Kiwi Ears.

So both the Kiwi ears are going for a midrange tonality while the IM4 is complimenting its open back design with an soundstage tuning.

Next, looking at the distortion, it is minimal even when playing loud:

And as a percentage, nothing too dramatic

Rating

I have given the Fosi Audio IM4 a Pragmatic rating of 4 out of 5. The IM4 offers some excellent value at a $109 MSRP (on sale for $99), with a good build, great comfort, some useful accessories but most of all an enjoyable ‘fun’ soundstagey tuning.

But, it’s not the last word in isolation or technicalities, and the semi‑open design won’t suit noisy environments.

Who is it for?

• Listeners who want a lively, musical v‑shape with controlled bass
• Anyone curious about an ‘open‑back’ IEM feel without spending much
• People who appreciates a quality build with some nice tuning options (nozzles + tips)

Who might skip it?

• Commuters needing high isolation
• Fans of strictly neutral tonality

Conclusion

The Fosi Audio IM4 is an easy recommendation in the sub‑€100 bracket for those chasing an open sounding IEM. It’s comfortable, well‑made, and tuned for maximum enjoyment without veering into harsh or boomy territory.

The open‑back design adds a hint of spaciousness you don’t usually get at this price, and the included nozzles/tips make it simple to tailor the sound slightly to taste. I feel this is a great IEM to add to a collection as it provides something unusual in this price range and I feel is well worth adding to your shortlist for a new IEM.