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For years, Fosi Audio has been the budget audiophile’s best friend, churning out those surprisingly capable little desktop amplifiers that sit on dorm room desks and office workstations everywhere. They built an empire on little black boxes that make speakers sing for pennies on the dollar. But with the IM4, the company is trying something entirely different. They aren’t just powering your sound anymore; they want to be the ones delivering it directly into your ear canal. The Fosi Audio IM4 is the brand’s debut in-ear monitor (IEM), and rather than playing it safe with a generic closed-back design, they’ve swung for the fences with an “open-back” architecture. It is a bold, weird, and surprisingly effective first attempt that brings a breath of fresh air – literally – to the crowded $100 market.
TABLE OF CONTENTS:
Positioning
The sub-$100 IEM market is a bloodbath of competition, dominated by giants like Moondrop and heavy hitters like Truthear. Launching your first product here is like walking into a knife fight with a spoon. Fosi’s strategy with the IM4 is clever because it refuses to fight on the same terms. By marketing this as an “open-back” IEM, they are carving out a specific niche for listeners who hate that clogged-up, underwater feeling of traditional sealed earphones.
Priced at $99, the IM4 sits in a dangerous middle ground – expensive enough to demand quality, but cheap enough to be an impulse buy for the curious. It isn’t trying to be the most analytical monitor for mixing engineers. Instead, it positions itself as a fatigue-free, spacious daily driver for people who want their music to feel like it’s happening in a room, not just inside their skull.
Read also: Fosi Audio i5 Open-Back Planar Magnetic Headphones review: Planar That Destroys The Competition
Design and build quality
If you have handled Fosi’s amplifiers, the IM4 will feel immediately familiar. The shells are machined from 6063 CNC aluminum alloy, giving them a cold, premium industrial feel that plastic competitors just can’t match. They are lightweight but dense, feeling like a serious piece of hardware rather than a toy. The defining feature, of course, is that grille on the back. It isn’t just aesthetic; it’s a functional vent that lets the 10mm dynamic driver breathe.
In the hand, they feel robust. The company includes a decent spread of accessories, including a carrying case and a silver-plated oxygen-free copper cable that feels pliable and resists tangling. Perhaps the most “audiophile” touch is the inclusion of interchangeable tuning nozzles. You get aluminum and brass options that screw onto the earphone to subtly tweak the frequency response. It is a fidgety but welcome addition that adds a layer of customization usually reserved for more expensive sets.
However, the open design comes with a caveat that you need to accept immediately: these do not isolate well. If you wear them on a noisy subway or a busy street, the world is going to leak in. While they aren’t as completely open as planar magnetic headphones, they block out significantly less noise than a standard IEM. This is a feature, not a bug, but it limits where you can use them.
Read also: SIMGOT EG280 Review: Budget IEMs for Gamers and Audio Enthusiasts
Sound
The sound of the IM4 is all about space. The open-back design effectively kills the pressure buildup that makes other IEMs fatiguing over long sessions. The result is a soundstage that feels expansive and airy, with instruments floating around your head rather than being injected into your brain stem. It is a “big” sound, one that makes live recordings and orchestral tracks feel properly atmospheric.
Tonally, Fosi has opted for a “fun” tuning rather than a strictly neutral one. The 10mm beryllium-coated driver delivers a bass response that is punchy and clean, though it won’t rattle your teeth with sub-bass rumble. It is polite bass – agile and textured, perfect for jazz or rock, but perhaps a bit light for die-hard hip-hop heads who want that visceral thump. The midrange is the star here, with vocals sitting forward and natural. Male voices have a pleasing thickness, while female vocals remain organic without getting shouty.
The interchangeable nozzles offer subtle shifts – the brass adding a bit more warmth – but the core character remains spacious and relaxed.
Read also: Kiwi Ears Airoso Review: 5-driver IEM Headphones for Audiophiles at an Affordable Price
Verdict
The Fosi Audio IM4 is a fascinating debut that proves the company understands audio is about more than just wattage. They haven’t just rebranded a generic OEM earphone; they have built something with a distinct point of view. It is an IEM for people who usually hate IEMs – offering the comfort and spaciousness of open-back headphones in a package that fits in your pocket.
It isn’t perfect. The lack of isolation makes it a poor choice for commuters, and bassheads might find the low-end too polite. But for $99, you are getting impeccable build quality, a unique open soundstage, and a listening experience that is genuinely refreshing.