When we usually hear the name Fosi Audio, we usually associate them to those affordable but overdelivering amplifiers and DACs for your home theatre or speaker systems.
But when I realized they came out with an IEM, I was quite intrigued and was interested to see how will they present themselves among this competitive scene.
The IM4 retails for 100USD, and yet, its asking price does not reflect what it’s capable of, because it does more than what you’d expect.
***Reviewer’s Note***
Thanks to Fosi Audio for sending these for evaluation. As always, this will be an honest review.
Specifications
Driver : 10mm N52 Dual Magnet Dual-Cavity Dynamic Driver
Diaphragm : PU Berylium-coated Diaphragm
Impedance : 21 Ohms
Sensitivity : 109dB
Cable : 4-core 392 strand 5N OFC SPC
Jack : 3.5mm only
Shell Material : 6063 CNC Aluminum Alloy
Unboxing & Accesories
The unboxing is fairly straight forward as with usual IEMS, I particularly like that the outer packaging is extremely simple in terms of presentation.
The Fosi IM4 provides three sets of tips : Balanced, Bass, and Deep Bass. It also provides another set of nozzles which we will get into at the sound segment.
Design and FIt
I like that even in the IM4, you can see Fosi Audio’s design language all over the lightweight CNC-ed aluminum alloy shell. With a matte black finish and an orange accented grill underneath the faceplate, this shells looks subtle but feels extremely exquisite.
In terms of fit, I personally do not find any issue throughout testing the IM4. The lightweight shell overall provides an even weight distrubution from the outer ear to the inner canal. I’ve worn this for hours on a single session and never have I felt any discomfort.
How Does It Sound?
Tested with :
- Hiby R6 iii 2025
- Hidizs AP80 Pro Max
- ifi GO BAR Kensei
- KAEI TAP-2
- MacBook Pro M3
The Fosi IM4 was tested for a cumulative estimate of 20 hours as of writing this review.
For reviewing’s sake, the entirety of the IEM is reviewed with the black nozzle. There will be another segment which covers the sonic implications of the gold nozzle.
General Impressions :
The initial thing that immediately caught my attention was the spacious presentation of the IEM. The IM4 is adopting an open-back design, and they have executed it well with good venting, where the IEM provides a seal yet does not let you hear too much of your surroundings outisde.
Sonically, the IM4 sounds balanced with quick and good response, and has a more soothing yet clear timbre and resolution
Bass :
- Clean, quick, spacious
- With its open back design, everything in this frequency range presents itself in a very freeing manner – filling up the big area and decay/extensions provides a good gradient into the wide staging.
- Subbass is of a balanced set – sufficient in terms of amplitude with a clean rumble
- Lower and midbass provides good dynamics that bounce and textural response to it – best to be heard with Stromae’s Fils de joie throughout
- 808s from hip-hop tracks are the best representation of the lower frequency range – thumpy, airy, and overall giving you a very clean recreation of the 808’s characteristics
- Kick drums sound huge and provides ample amount of diffusion
Vocals :
- Positioning of the IM4 is ideal, in the same plane as most instruments
- Tonality and timbre of vocalists are quite natual
- Male vocals overall have good thickness
- Female vocals come off with a slight hoarser response which adds to a more organic approach
- No instance of sibilance
Instruments and Midrange :
- Engaging, musical, detailed
- Instruments on the IM4 benefit a lot with its microdetails that constantly complements its macros, from finger grazes on strings, to the very light thuds and blunted taps on keys prior to the notes being played
- Strings showcase the detail retrieval the best with this set – providing the sonic tactility that keeps you constantly engaged
- Vocal ensembles are also extremely dreamy and fulfilling
Upper-Mids and Treble :
- Upper-mids and treble overall have a realistic amount of extension, does not overdo it
- No instance of uncomfortable peaks in the slightest. Matter of fact for me, this one is leaning towards a laid-back pair
- Percussions sound clean and clear, and have a light extension accompanying every cymbal/ride/hat strike
- Presentation of instruments on this region are on the brighter side, but still has some slight smoothening to not make it feel fatiguing
Technical Highlights :
Dynamics of the IM4 is quite coherent even out of the box. On lower volumes you tend to hear more details, and on normal volumes the musicality kicks in, accompanying the details
Staging is wide and provides an absurd feeling of width and depth. On the far ends of both L and R, there is no distortion or loss of resolution when instruments or details are presented there.
Imaging and seperation are done with good three-dimensional presence. You can feel things are spaced and based on 3 different axes – which is hard to come by as most IEMs tend to just have imaging more in 2 axes
Detail retrieval is also another strong suit of this IEM. The interesting part is the IM4’s was of pushing details in the form of what comes after the macros. Overall quite an analytical pair
But wait, there’s more!
The IM4 also provides a seperate gold nozzle, which mainly tilts the FR to feel more bright and the mids and up feeling more elevated in amplitude.
With this change you can feel the entire context of the IM4 leans itself to feel more neutral ever so slightly. Bass in the IM4 also feels more diffused, and toned down ever so slightly.
Final Thoughts
All in all, the IM4 is definitely an enjoyable IEM for the likes of many. Usually I’ll be having some nitpicks here and there, but with this? I don’t think there’s any I’d complain about. For 100USD, you really can’t go wrong with this set that articulates instruments well, with good complement of details.
