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

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

The Fosi Audio IM4 is Fosi’s first IEM – and a bit atypical. Open-back architecture in 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 dynamic driver with PU beryllium coating, CNC-machined 6063 aluminum shell, two interchangeable nozzles and nine sets of ear tips in three tuning profiles. Built for desktop listening, not for travel.

Open-back architecture

The rear chamber of each earpiece has acoustic vents that allow the driver to breathe out into the open air instead of into a sealed cavity. This eliminates the pressure buildup and reflections that give closed IEMs their characteristic “sound inside the head” presentation. The trade-off is isolation – it doesn’t exist. Sound leaks out in both directions, which makes the IM4 a poor choice for public transport or noisy offices, but an excellent option for home listening at a desk.

The soundstage benefit is real and consistently noted in reviews: the width and depth are noticeably greater than what closed-back IEMs at this price deliver. The imaging is precise enough to be useful in gaming, particularly for detecting directional cues in FPS.

Driver and Acoustic Design

The 10mm driver combines a PU (polyurethane) diaphragm with a beryllium coating. The PU provides mass and compliance for a natural, smooth low-frequency response; the beryllium layer adds rigidity and speed in the high frequencies, extending clarity without adding harshness. Dual N52 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 entire sound range. The rear chamber is filled with acoustic damping material to suppress resonance peaks that would otherwise color the presentation with harshness or ringing.

Interchangeable 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 advances the mids and adds clarity in the highs – better for detail-oriented listening and gaming, where that slight presence at 5kHz helps footsteps and positional cues stand out. This is a genuine functional tuning system, not a cosmetic one.

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

Construction and handle

The shell is machined from 6063 aluminum alloy using a 5-axis CNC process, sandblasted and anodized for a smooth, pleasant-to-the-touch finish. At 7g per earpiece, it’s light enough for hours of fatigue-free use. The cable is silver-plated OFC 5N copper with 4 colors and 392 strands, braided, terminating in a 3.5mm gold-plated jack. The 2-pin 0.78mm connector supports aftermarket cable upgrades, including balanced 4.4mm terminations.

Team opinion

The IM4 is a genuinely interesting first. Fosi didn’t launch a neutral, safe IEM with a closed design – it went open-back with interchangeable tuning. For a first IEM, that’s a strong sign of intent. The result is an IEM that sounds wider and more natural than most closed competitors at this price, at the cost of an isolation that closed-design users may not be willing to do without.

For desktop use alongside the K5 Pro, K7 or DS3 – which is exactly the context for which Fosi designs these products – the IM4 makes sense. The open-back soundstage goes well with a neutral DAC, and the nozzle system allows you to fine-tune the character to what you’re listening to. For desktop gaming, the wide soundstage and that mid-range presence make the positional audio noticeably better than most closed-back IEMs at this price.

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

⚠️ Isolation Warning: The open-back design does not provide passive isolation. Ambient noise enters freely and sound leaks out. Designed for quiet, controlled listening environments.

IN THE BOX

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