Fosi Audio Introduces IM4: HiFi In-Ear Monitors for Everyday Listening, Delivering Unprecedented Value

Fosi’s first-ever in-ear monitors feature an open-back design and customizable fit, delivering natural sound and comfort at an economical price point

September 30, 2025

Shenzhen, China, September 30, 2025 — Fosi Audio, a leading manufacturer of affordable hi-fi audio products, announces the IM4, the company’s first-ever in-ear monitors. Designed with an open-back architecture and a lightweight CNC aluminum shell, the IM4 brings spacious, natural sound and all-day comfort to the portable HiFi category - all within an attractive and economical price point.

“The IM4 is a milestone in our journey of earphone development, our first step in this category made with sincerity and care,” said Ryan Huang, Founder and CEO of Fosi Audio. “With its natural, open sound and lightweight comfort, the IM4 is ideal for everyday listening, whether you are unwinding at home or discovering detail on the go. The name IM4 resonates with the phrase ‘I am for,’ a declaration of support and identity that reflects our spirit of openness and our commitment to listening closely to the feedback and ideas of listeners worldwide.”

Hear the unheard

The IM4 is built around a 10mm beryllium-coated diaphragm driver paired with an N52 dual-magnet system and dual-chamber acoustic design. Together, these elements deliver smooth highs, detailed mids, and deep, controlled bass with a natural balance suited for music, movies, gaming, and streaming. The open-back architecture expands the soundstage far beyond typical IEMs, giving listeners an experience that feels closer to full-size headphones while remaining lightweight and portable.

Each earpiece is CNC-machined from aluminum and weighs just 7 grams, providing a secure, pressure-free fit that adapts to a wide range of ear shapes. The ergonomic housing and breathable design support all-day listening at home, on the move, or at work.

Customization options for everyone

The IM4 also offers extensive customization. Three sets of ear tips, tuned for balanced, bass, and deep bass, let listeners adjust the sound signature while maintaining comfort and isolation. Swappable aluminum and brass nozzles provide additional tuning options, from smoother highs to more energetic bass and treble. This flexibility allows the IM4 to adapt not only to different genres but also to different preferences, from relaxed, detailed listening to powerful, bass-driven sessions. Detachable cables in both 3.5mm and 4.4mm versions add further versatility, making it easy to pair with a wide range of devices.

Specifications

Driver: 10mm beryllium-coated diaphragm
Magnet: N52 dual-magnet system
Acoustic design: Dual-chamber, open-back
Housing: CNC-machined aluminum
Weight: 7g per earpiece
Tuning options:
- Interchangeable aluminum and brass sound nozzles
- Three sets of ear tips: balanced, bass, and deep bass
Cables: Detachable 3.5mm and 4.4mm connections
Frequency response: 20Hz–20kHz
Impedance: 21Ω
Sensitivity: 109dB/mW