The Fosi IM4 is an in-ear monitor from Fosi Audio featuring an open-back, single dynamic driver design. It incorporates dual acoustic chambers along with a dual magnet, dual cavity driver construction. The IM4 uses a beryllium-plated polyurethane diaphragm, a popular choice in IEM drivers due to beryllium’s high rigidity and low weight. Launch price is 99 USD
FOSI IM4 SPECIFICATIONS
- Type: Open back IEM
- Driver: 10 mm dynamic driver
- Type: Dual-Magnet Dual-Cavity (N52 magnets)
- Diaphragm: PU Beryllium-coated Diaphragm
- Sensitivity: 109dB
- Impedance: 21 Ohms
- Frequency Response: 20Hz-20kHz
- Cable: 4-core 392-strand 5N OFC Silver-plated Copper
- Cable connector: 2-PIN 0.78mm
- Cable termination: 3.5 mm gold-plated plug
- Shell Material: 6063 CNC Aluminum Alloy
- Weight (per earbud): 7g
- Weight (incl. cable): 38g
Current price and availability:
Disclaimer: Fosi sent us the review unit in exchange for an unbiased review.
BUILD AND COMFORT
The IM4 features shells made from CNC machined aluminium, with each earpiece weighing just 7 g. The cable is light and flexible, made of silver-plated OFC copper. It connects to the earphones via detachable standard 0.78 mm 2-pin connectors and terminates in a standard 3.5 mm unbalanced jack.
The package includes several tuning options. There are two sets of tuning nozzles, brass and aluminium, each offering a slight variation in sound. According to Fosi, aluminium nozzles provide reduced sibilance and added bass, while brass nozzles deliver clearer mids and treble.
The IM4 also comes with three different types of ear tips: Balanced, Bass, and Deep Bass, each delivering varying degrees of bass quantity as their names suggest. All three types are available in three sizes.
All these tuning options are a welcome addition for the end user, offering plenty of possibilities to fine-tune the sound to your liking.
As a reviewer, however, I will not dig into all the possible combinations. First, it would simply be too much work, and second, tips and nozzles interact with our ear canals in a way that makes their effect on sound a very personal and individual matter.
I will review the earphones with the pre-mounted nozzles, using, as I always do, my own selected third-party ear tips. Getting ear tips right is very important, and I have my own selection that I know provides me with a good seal and comfort.
I also have some experience regarding how these ear tips typically affect the sound for me personally, which gives me a better reference point for reviews than trying completely different tips every time. Also, most experienced IEM users will deploy their own preferred tips anyway.
LISTENING IMPRESSIONS
Related equipment:
- DAC/AMP: Ferrum Erco mk2
- Tips: Moondrop Spring Tips and Tangzu Tang Sancai Wide
Santa Marta by Louis Matute
Excellent rendition. Spacious and balanced with a nice, warm bass. Even though it is not overly crisp or bright, there is a sense of dryness, or rather a dry warmth, which I like.
Moo)n by Lindström
Again, there is this sense of dry warmth. The bass is punchy, the percussion textured, and the soundstage spacious.
Dancing in the Dark by Jørgen Dahl Moe
Mr. Moe absolutely has my favourite interpretation of this song. It sounds very good on the IM4, though the vocals have more dryness to them than I prefer, although this is mainly down to the production rather than the earphones.
As Before by Olga Konkova
The cajón drum has a nice punch, though not earth-shattering. The piano is unusually smooth, and the vocals are quite textured.
Meslange de musique latine, françoise et italienne: Ariette dans le goût nouveau by Sophie de Bardonnèche
Very airy with great textures. Again there’s this appealing sense of “dry warmth”.
Outlaws by Bill Frisell
Again it sounds airy, spacious and with excellent texture – especially in the mids and highs.
Cloud Wars by Medeski, Martin, and Wood
The strong bass is nicely textured and not overpowering.
Woodpecker by Tingvall Trio
The percussion is very snappy, the piano is gentle and liquid sounding.
Sandermosen by Maridalen
The saxophones and organ sound alive, with texture and nuance, as do the double bass – here played with a bow.
What He Wrote by Laura Marling
Vocals are warm and intimate, the plucked guitar is organic and blends in well with the rest.
Almost Like The Blues by Leonard Cohen
The bass is organic and textured, with warmth and body, as is the vocals.
Escape Route by Boris Blank
Here too, the bass has an organic quality, the trumpet clear and balanced without being excessively crisp. Again, there is this sense of dry warmth to the timbre.
Young Vivaldi by Modo Antiquo
Excellent timbre. Not too crisp, not too laid back. Textured and rich in nuance, again with that dry, warm character mentioned several times before.
Summer 3 Vivaldi Recomposed by Max Richter
Great separation and layering. It leans dark rather than bright, yet avoids sounding crowded or muffled as many earphones do. Very good.
Vicarious by Tool
Excellently balanced between tight and full-bodied. Energetic but not fatiguing, with great separation. Not harsh, not bloated.
WRAPPING IT UP
Sound Signature
I find the sound signature to be quite neutral in a natural way, with bass that is certainly present but not overwhelming.
Treble
The treble is not overly sharp or crisp, but carries a slight dryness to it. I find it nicely balanced and non-fatiguing.
Midrange
The midrange is nicely present, with plenty of texture and nuance while retaining warmth and intimacy. As mentioned many times, there is a somewhat “dry warmth” to the timbre of instruments in general, a characteristic I quite like.
Bass
The bass has body and organic presence while also remaining tight and textured.
Soundstage and Imaging
The soundstage is very large and open. The imaging is good but not the most precise or pinpointed I have experienced. In other words good, but not exceptional.
Detail, Dynamics, and Timbre
I find the detail level to be good for the price, and dynamics are excellent. As for the overall timbre, as mentioned, I find it best characterised as moderately warm with a dry tint.
COMPARISONS
Associated equipment:
- Amplifier: Topping A90
- DAC: RME ADI-2 DAC FS
SIVGA QUE vs FOSI IM4
The Sivga Que is a single driver, wooden back IEM with a 10 mm single magnet dual cavity driver. The diaphragm is, like the IM4, beryllium coated. The Que was launched in 2024 and is currently priced at 69 USD.
The Fosi IM4 and Sivga Que sound very similar, sharing a close tuning. The IM4 offers slightly more bass, better texture, and stronger dynamics, making it technically superior and worth its higher price. That said, both are excellent, and the Que can sometimes be more enjoyable – it’s often about subjective preference.
You can read my full comparison review here:
SIMGOT EA500 vs FOSI IM4
The Simgot EA500LM is a single dynamic driver earphone that launched early in 2024, featuring a 10mm lithium-magnesium dome diaphragm. It’s currently priced at 89 USD.
Comparing the EA500LM and IM4, both are excellent but with different presentations and strengths. The IM4 is likely the better all-rounder and handles complex passages with more ease. The EA500LM excels with simpler acoustic tracks where it can sound more organic and dynamic.
You can read my full comparison review here:
AMPLIFICATION
I tried the Fosi IM4 with a few desktop amplifiers and several portable options, including Fosi’s own DS3. The different sources all drive the IM4 confidently, though with some variation. Overall, the IM4 seems very easy to drive and flexible with regard to sources.
CONCLUSION
The Fosi IM4 is a well-tuned IEM with great technical performance, delivering excellent value for its price and serving as a capable all-rounder. Its characteristic dry, warm timbre is very appealing; not unnatural, merely a subtle trait, but one that makes the IM4 stand out in a crowded market.
I really liked Fosi’s first headphone, the i5, and the IM4 is an excellent IEM debut. I am very much looking forward to what Fosi has to offer in the future.
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