In this video I reviewed the Fosi Audio IM4, which is Fosi’s first ever IEM—and it’s a pretty quirky debut at around $100. It’s a single dynamic driver set with an “open-back” style cutout, an all-metal shell, and a surprisingly complete accessory pack: a PU pouch, three tip sets (balanced, bass, deep bass), and two nozzle options (the stock black nozzles and the brighter brass/gold nozzles). I also walked through comfort and build, and I talked about why I don’t really buy “open-back IEM” as anything more than marketing most of the time. For sound, I tested mainly on my iBasso DX260 and the Fosi ZH3 DAC/amp. The IM4 is a bassy, warm V-shape that’s clearly aiming for a non-fatiguing, easy listen rather than max technical performance. Bass is the headline: deep, satisfying, and mid-bass slam-heavy, but not fast or super textured. Mids are full and smooth with rich male vocals, while treble is tamed and slightly rolled off—great if you’re treble-sensitive. I also compared it to the Kiwi Ears Airoso and the Etymotic ER2XR, then wrapped up with the main downsides (like the nozzles undoing themselves) and who I think this set is best for.