Build Quality

Overall build quality is solid. There is a very minor rattle when shaking the mouse. When applying lateral pressure, minor creaking but no flexing of the shell can be observed. The forward side button can be activated by squeezing right below it, but doing so requires an irregular application of force. Lastly, no accidental clicks occur when slamming down the mouse.

Buttons

Main buttons on the M4 Air are good to very good. Both pre and post-travel are moderate, resulting in a still quite firm and snappy button response. The buttons are visually separated from the shell and can be moved from side to side, but this needs to be provoked and isn't noticeable during regular use. Button stiffness is medium. A pair of Kailh GM 4.0 (60 M) switches is used here.


Side buttons are good. Both have some pre-travel, but post-travel is equally low on either. Button response is fairly snappy and firm. The buttons are fairly loud, however, which can be irritating. The pressure point is mostly even across the entirety of these. Button placement is good as actuation is possible very easily by rolling one's thumb across. Compared to the TUF M4 Wireless, these are quite a bit smaller. A set of low-profile switches from Kailh (yellow plunger) is used for these.

At the top of the mouse is a single button for cycling through the set CPI steps, which works just fine. A seemingly unbranded tactile switch (brown plunger) is used for this one.

Scroll Wheel

The scroll wheel is good. Scrolling up is noisy, but tactility is quite good, with clearly defined steps and a matching tactile feel when scrolling. The encoder comes from Kailh (black core) and has a height of 12 mm. The middle (scroll wheel) click requires medium force for actuation. A seemingly unbranded tactile switch (blue plunger) is used here.

Surface

The M4 Air has a slightly rougher matte surface all over, which provides great grip. In addition to that, the sides feature a triangle pattern, which adds further grip but does catch dirt rather easily. The surface doesn't attract fingerprints, dirt, or the likes too much, is easy to clean, and there are no signs of wear left after doing so. All in all, excellent materials.

Button Sound Test

Disassembly

Disassembling the M4 Air is easy. Remove the bottom foot and the two screws beneath it, and the top shell will come off with ease. The internal design is equally simple. Everything aside from the slightly thicker side-button PCB slotted into the left side panel and connected through a ribbon cable sits on the compact and very thin main PCB. The sensor has been rotated by 90 degrees to save precious vertical PCB space. Three screws in total are used to affix the PCB to the bottom. The MCU is an STM32L412C8U6, whose datasheet can be found here.


As for the soldering and general quality of the PCB, I'm unable to find any noteworthy flaws.