Build Quality

Overall build quality is solid. When shaking the mouse, a minor rattle comes from the main buttons. When applying lateral pressure, no creaking or flexing of the shell can be observed. Activating the side buttons by squeezing the sides is impossible. Lastly, accidental clicks do not occur when slamming down the mouse, unless the debounce time is set to minimum.

Buttons

Main buttons on the HSK Pro 4K are good to very good. The right button in particular has high pre-travel, whereas the left one has moderate post-travel, though button response is quite firm and snappy nonetheless. While the buttons are visually separated from the shell, lateral button movement is fairly low even when provoked. Button stiffness is medium to light. A pair of Zippy DF3-P1L1 (60 M) switches are used.


Side buttons are excellent. Both pre and post-travel are very low, resulting in a snappy and pleasing button response nonetheless. The pressure point is even across the entirety of these. Button size and placement are good, as actuation is possible rather easily by rolling one's thumb across. That said, due to how limited the space for one's thumb is, hitting the side buttons inadvertently is very much a possibility. A set of low-profile switches from Huano (white plunger) is used for these.

Unlike on the Hati-S Plus 4K/ACE, the on/off-switch at the bottom can be accessed with one's fingers, albeit not as easily as on the HTX 4K.

Scroll Wheel

The scroll wheel is very good. Due to the open bottom, noise levels are elevated, but tactility is great, providing very distinct steps and controlled scrolling. The encoder comes from TTC (gold) and has a height of 7 mm. The middle (scroll wheel) click requires high force for actuation. A seemingly unbranded tactile switch (black plunger) is used here. Interestingly, the scroll wheel isn't fully seated into the encoder, which is by design, as otherwise it would not be centered.

Surface

The HSK Pro 4K has a smooth matte surface all over. Grip is fine, and it doesn't attract fingerprints or dirt too much. It 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 HSK Pro 4K is easy. All skates and the screws beneath ought to be removed. The sides are additionally locked in with clips, which are dislodged quite easily. As always, wedging in a credit card (or similar) may help.

The internal design is very efficient. The side buttons sit on their own PCB vertically slotted into a plastic assembly that is part of the bottom shell and connected through a 2-pin connector. The battery sits on a foam pad glued to the bottom shell. The main buttons, scroll wheel encoder, and USB connector sit on their own PCB, and a secondary PCB layered above slots into the first one, mainly holding the sensor and MCU. Four screws are used to affix the main-button PCB to the bottom shell, along with an additional two for the sensor PCB. The MCU is a Nordic nRF52840 in the WLCSP variant consisting of a much smaller package, whose datasheet is found here. The Bluetooth capability of the chip remains unused.


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