Unboxing and build quality
The unboxing experience is essential yet refined. Once opened, the presentation is clean and organized. I really appreciated the included faux leather case: it’s well-made and perfect for carrying the IEMs, cable, and various accessories (like the interchangeable nozzles) without the risk of losing them. The construction conveys solidity: the CNC aluminum shell is robust and pleasant to the touch, while the included cable is of good quality, doesn't tangle, and is the correct length for desktop use. The orange grilles on the back aren't just an aesthetic nod to the brand, but functionally highlight the Open-Back nature of these monitors. In the ear, the IM4s feel surprisingly light. I tested them for long sessions (over an hour) without experiencing any fatigue: the ergonomics are excellent, and the wide selection of included ear tips makes these IEMs extremely versatile, allowing you to fine-tune both comfort and sound (with the balanced, bass, and deep bass variants).
A clarification on isolation is necessary: being open-back, they don't isolate from ambient noise at low volumes, and raising the volume results in some sound leakage. However, one shouldn't expect the "total transparency" typical of over-ear planar magnetics; the IM4s sit somewhere in the middle, offering a soundstage that is more open than classic in-ears, while still maintaining a certain intimacy and sound pressure.
The listening test
For the test, I used two different configurations: a mobile setup with Apple Music (Hi-Res Lossless) connected via a JCALLY JM12 dongle to a phone/laptop, and a desktop setup with a Raspberry Pi 3B+ (DAC HAT and Volumio) streaming FLAC files.
The first sensation is one of great cleanliness and precision: thanks to the open design, there is a lot of air between the instruments, separation is sharp, and I never perceived congestion in the mix, with remarkable detail retrieval.
I thoroughly compared the two configuration options offered by the interchangeable nozzles (using only the balanced tips):
Aluminum Nozzles (My Choice): This is the stock configuration and the one I preferred for a more balanced output. Here, the bass fully convinced me: it is tight and precise. There is the right amount of punch—controlled and fast. The highs are airy and crystalline, present enough without ever becoming sibilant, and the midrange feels correct and never recessed. It is the balance I found most coherent.
Brass Nozzles: These don't completely overhaul the sound but redefine its character. The upper-midrange gains more clarity, giving a sensation of more "bite" in the higher frequencies. However, I noticed that in this configuration, while the bass maintains good impact, it tends to slightly lose that surgical precision I appreciated with the aluminum. It is likely a tuning suited for specific genres, but personally, I find the aluminum setup more complete.
Conclusions
The Fosi Audio IM4s are a pleasant surprise. They stand out from the crowd thanks to a design that offers spatiality and a tuning that aims for precision rather than exaggerated, overpowering bass. They are versatile and well-built headphones, suitable for those seeking a clean and detailed listening experience, with the added value of being able to experiment thanks to the interchangeable filters—all at a truly competitive price-to-performance ratio.
The unboxing experience is essential yet refined. Once opened, the presentation is clean and organized. I really appreciated the included faux leather case: it’s well-made and perfect for carrying the IEMs, cable, and various accessories (like the interchangeable nozzles) without the risk of losing them. The construction conveys solidity: the CNC aluminum shell is robust and pleasant to the touch, while the included cable is of good quality, doesn't tangle, and is the correct length for desktop use. The orange grilles on the back aren't just an aesthetic nod to the brand, but functionally highlight the Open-Back nature of these monitors. In the ear, the IM4s feel surprisingly light. I tested them for long sessions (over an hour) without experiencing any fatigue: the ergonomics are excellent, and the wide selection of included ear tips makes these IEMs extremely versatile, allowing you to fine-tune both comfort and sound (with the balanced, bass, and deep bass variants).
A clarification on isolation is necessary: being open-back, they don't isolate from ambient noise at low volumes, and raising the volume results in some sound leakage. However, one shouldn't expect the "total transparency" typical of over-ear planar magnetics; the IM4s sit somewhere in the middle, offering a soundstage that is more open than classic in-ears, while still maintaining a certain intimacy and sound pressure.
The listening test
The first sensation is one of great cleanliness and precision: thanks to the open design, there is a lot of air between the instruments, separation is sharp, and I never perceived congestion in the mix, with remarkable detail retrieval.
I thoroughly compared the two configuration options offered by the interchangeable nozzles (using only the balanced tips):
Aluminum Nozzles (My Choice): This is the stock configuration and the one I preferred for a more balanced output. Here, the bass fully convinced me: it is tight and precise. There is the right amount of punch—controlled and fast. The highs are airy and crystalline, present enough without ever becoming sibilant, and the midrange feels correct and never recessed. It is the balance I found most coherent.
Brass Nozzles: These don't completely overhaul the sound but redefine its character. The upper-midrange gains more clarity, giving a sensation of more "bite" in the higher frequencies. However, I noticed that in this configuration, while the bass maintains good impact, it tends to slightly lose that surgical precision I appreciated with the aluminum. It is likely a tuning suited for specific genres, but personally, I find the aluminum setup more complete.
Conclusions
The Fosi Audio IM4s are a pleasant surprise. They stand out from the crowd thanks to a design that offers spatiality and a tuning that aims for precision rather than exaggerated, overpowering bass. They are versatile and well-built headphones, suitable for those seeking a clean and detailed listening experience, with the added value of being able to experiment thanks to the interchangeable filters—all at a truly competitive price-to-performance ratio.
