Fosi Audio IM4 – Open-Back IEM, 10mm Beryllium Driver, Swappable Nozzles

Fosi Audio IM4 – Open-Back IEM, 10mm Beryllium Driver, Swappable Nozzles

The Fosi Audio IM4 is Fosi's first IEM — and an unusual one. Open-back architecture on an in-ear is not the obvious choice for a debut product, but it defines everything about how the IM4 sounds and who it's for. Single 10mm beryllium-coated PU dynamic driver, CNC-machined 6063 aluminum shell, two swappable nozzles, and nine ear tip sets in three tuning profiles. Built for desktop listening, not commuting.

Open-Back Architecture

The rear chamber of each earpiece has acoustic vents that allow the driver to breathe into open air rather than into a sealed cavity. This eliminates the pressure buildup and reflections that give closed IEMs their characteristic "in-your-head" presentation. The trade-off is isolation — there is none. Sound leaks both ways, which makes the IM4 a poor fit for public transport or noisy offices, but an excellent choice for home listening at a desk or alongside a DAC setup.

The soundstage benefit is real and consistently noted across reviews: width and depth are notably larger than what closed-back IEMs at this price deliver. The imaging is precise enough to be useful in gaming, particularly for directional cue detection in FPS titles.

Driver and Acoustic Design

The 10mm driver pairs a PU (polyurethane) diaphragm with a beryllium coating. PU provides mass and compliance for natural, smooth low-frequency response; the beryllium layer adds stiffness and speed to the high frequencies, extending clarity without adding harshness. N52 dual magnets generate a strong, stable magnetic field for fast transient response and low non-linear distortion — rated at THD below 1%.

The dual-chamber design separates front and rear acoustic behavior. The front chamber uses nine ports for balanced frequency output across the sound range. The rear chamber is packed with acoustic damping material to suppress resonance peaks that would otherwise color the presentation with harshness or ringing.

Swappable Nozzles and Tuning Tips

Two nozzle options are included. The aluminum nozzle produces a warmer, smoother character — better for long listening sessions and genres where midrange texture matters. The brass nozzle brings forward mids and adds treble clarity — better for detail-oriented listening and gaming, where that slight 5kHz presence helps footsteps and positional cues cut through. This is a genuine functional tuning system, not cosmetic.

Nine ear tips in three profiles complement the nozzle system. Balanced tips maintain the intended frequency response. Bass tips add low-end emphasis. Deep Bass tips extend sub-bass presence. Tip and nozzle combinations allow meaningful customization of the final signature without additional purchases.

Build and Cable

The shell is machined from 6063 aluminum alloy using a 5-axis CNC process, sandblasted and anodized for a smooth, skin-friendly finish. At 7g per earbud, it's light enough for hours of wear without fatigue. The cable is 4-core 392-strand 5N OFC silver-plated copper, braided, terminating in a 3.5mm gold-plated plug. The 2-pin 0.78mm connector supports aftermarket cable upgrades, including 4.4mm balanced terminations.

Gamer Unlocked Take

The IM4 is a genuinely interesting debut. Fosi didn't launch with a safe, neutral tuned closed-back — they went open-back with swappable tuning. For a first IEM, that's a strong signal of intent. The result is an IEM that sounds wider and more natural than most closed competitors at this price, at the cost of isolation that closed-back users may not be willing to give up.

For desktop use alongside the K5 Pro, K7, or DS3 — which is exactly the context Fosi designs these products for — the IM4 makes sense. The open-back staging pairs well with a neutral DAC, and the nozzle system means you can tune the character to what you're listening to. For gaming at a desk, the wide soundstage and that mid-treble presence make positional audio notably better than most closed IEMs at this price.

Honest note: start with the aluminum nozzle and deep bass tips. The brass nozzle with wide-bore tips can push the treble into fatigue territory for some listeners. And if you listen on the move or in noisy environments, this is not the right IEM — the lack of isolation is a real limitation.

⚠️ Isolation Notice: Open-back design provides no passive isolation. Ambient noise enters freely and sound leaks outward. Designed for quiet, controlled listening environments.

IN THE PACKAGE

• 1× Fosi Audio IM4 IEM pair
• 1× 5N OFC Silver-plated Braided Cable (3.5mm)
• 2× Swappable Nozzles (Aluminum and Brass)
• 9× Ear Tips (Balanced, Bass, Deep Bass — S/M/L each)
• 1× Carrying Case