What Makes the Fosi Audio IM4 So Different from Every Other IEM?

A Fresh Approach to Portable Sound

The Fosi Audio IM4 introduces something refreshingly different to the in-ear monitor (IEM) world — a design that merges the airy sound of open-back headphones with the precision and portability of in-ear monitors.

In his video review, Gamesky praised Fosi Audio for bringing this hybrid idea to life. It’s a creative product from a trusted brand — and one that captures attention for both its sound signature and design philosophy.

Design that Balances Simplicity and Style

The IM4 embraces a sleek, understated aesthetic with a matte black or silver finish, accented by a subtle open-back grill. Its design is modern yet minimalist — appealing to those who appreciate sophistication without flashiness.

Inside the box, Fosi Audio includes a range of thoughtful accessories:

• Two interchangeable sound nozzles (aluminum and brass) to tailor the tonality.

• Three sets of ear tips that cater to different listening preferences — balanced, bass-focused, and deep bass.

• A stylish blue pleather hard case for protection and portability.

The IM4 uses a 10mm beryllium-plated dynamic driver and a detachable silver-plated copper cable with a 2-pin connection and standard 3.5mm jack. Every element feels carefully designed to provide both quality and customization.

Technical Snapshot

Gamesky notes that while the IM4 is efficient enough to run from most devices, pairing it with a portable DAC or amp can further enhance dynamics and clarity — allowing listeners to fully appreciate its sonic depth.

Sound Performance: Spacious and Lively

One of the IM4’s standout qualities is its spatial soundstage. Thanks to its open-back design, the earphones deliver a sense of air and dimension rarely found in traditional IEMs. Vocals and instruments feel naturally placed, with a presentation that brings music closer to a live performance experience.

Bass

The bass is clean, well-textured, and responsive, offering a tight foundation that complements rather than overpowers the rest of the mix. It maintains energy and rhythm while preserving the IM4’s open, airy character.

Mids

Vocals and acoustic instruments are rendered with impressive clarity and realism. The mids have a natural tone that works beautifully for genres focused on voice or melody.

Treble

High frequencies are clear and detailed, providing brightness and definition without fatigue. With the brass nozzle, the treble gains extra sparkle, while the aluminum nozzle introduces a slightly warmer balance for smoother listening sessions.
Gamesky mentioned his favorite setup was the aluminum nozzle combined with deep bass ear tips, which provided a balanced sound full of detail and depth.

The IM4’s open acoustic design offers listeners a sense of space that feels more immersive than typical closed IEMs — yet it remains practical for everyday use, with moderate sound leakage and comfortable isolation.

With a modest 21Ω impedance, the IM4 is easy to drive from portable sources, making it equally suited for casual listening or audiophile exploration. Its sound presentation feels natural, engaging, and delightfully different from most earphones in this price range.

A Fresh Perspective on Portable Audio

In his closing thoughts, Gamesky commended Fosi Audio for introducing something genuinely new to the IEM market. The IM4 stands out for its innovative design, versatile tuning, and refined presentation that emphasizes musical space and balance.

While those seeking a heavily bass-boosted sound might prefer other tuning styles, the IM4 focuses on clarity, openness, and realism — qualities that audiophiles often prize above all.

The IM4 is a thoughtfully crafted IEM that dares to experiment and succeeds. Its blend of open-back spaciousness and portable practicality makes it one of the most interesting releases of 2025.

For users who value natural tonality, live-stage imaging, and flexible tuning, the IM4 offers exceptional value at around $100 — proof that innovation in personal audio is very much alive.