Fosi Audio IM4 Open-Back IEMs Under $100 with Customizable Tuning and Fit

First teased at SoCal CanJam 2025, Fosi Audio is doubling down on 2025 momentum with a string of recent releases—the i5 headphones, ZH3 DAC/Amp, Luna3 turntable—and now its first-ever foray into in-ear monitors with the IM4. At under $100, these open-back IEMs with a customizable fit are an eyebrow-raiser in a market currently obsessed with mid-tier and stratospheric high-end models from Campfire Audio, Noble, FiiO, and an endless parade of boutique Asian brands.

Sure, $2,000 IEMs exist, but most consumers aren’t exactly itching to drop that kind of cash, and wireless earbuds dominate wallets and pockets. Yet for the loyal wired IEM crowd, the IM4 could be the surprisingly accessible “high-end” fix they’ve been waiting for—without the mortgage.

Fosi IM4 Core Features

Starting with an open-back design, the IM4 incorporates solid features:

• Beryllium Diaphragm: A 10 mm PU Beryllium-coated diaphragm delivers smooth mids, punchy bass, and crisp high frequencies for a natural, detailed sound.
• Dual Magnet: Supported by an N52 Dual-Magnet assembly, the speaker drivers maintain a stable magnetic field, providing precise control, fast response, and wide dynamic range.
• Dual Chamber: The acoustic design features a front chamber with nine ports for balanced frequencies, while the rear chamber enhances midrange detail. Acoustic damping material in the rear chamber absorbs standing waves that can cause distortion and harshness.
• OFC Cable: The IM4 includes an OFC (Oxygen-Free Copper) silver-plated cable, ensuring fast, stable signal transmission and shielding against interference for cleaner sounding audio.

From Ryan Huang, Founder and CEO of Fosi Audio: “The IM4 is a milestone in our journey of earphone development, our first step in this category made with sincerity and care,….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.”

IM4 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
• 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

The Bottom Line

The IEM product category is very competitive, but most entries are on the more expensive side, such as the Beyerdynamic 7x IE ($579), Campfire Audio’s Alien Brain ($999), or the stratospherically priced Noble Kronos ($4,500). 

Priced at just $99, the Fosi IM4 offers an enticing entry into the wired IEM world with features often found at higher price points. While it faces competition from the Meze Audio ALBA ($159), DUNU x KOTO ITO ($199), and Campfire Audio Axion ($249), the IM4 stands out for delivering open-back, beryllium-driven sound and a customizable fit at a fraction of the cost.

Success here could signal Fosi expanding further into the IEM category—and perhaps even dipping into wireless earbuds—making them a brand to watch for affordable, high-quality audio options.

Where to buy: $99 at Fosi Audio in black or silver.

Related Reading

• Beyerdynamic’s DT Series IEM Offers Different Sonic Profiles for Engineers and Musicians: NAMM 2025
• AFUL Dawn-X IEMs: ‘Awful’ Name, But 1DD+8BA+4EST+1BCD Flagship Power
• Noble Van Gogh IEM: Four-Driver Hybrid Meets Starry Night—No Two Look the Same (Good Luck Picking!)
• FiiO’s FX17 Flagship IEMs Integrate Electrostatic, Dynamic, and BA Drivers