Most NAS-specific cases (Synology, QNAP, TerraMaster) use for the mainboard and M4 for drive backplanes. Generic “PC standoff kits” often lack M4, so check your chassis manual.
One pro tip: Thread a spare screw into the standoff before installing it into the case. This gives you leverage and prevents over-tightening. nas standoffs
I installed a Supermicro X11SSH-F board into a Chenbro NR40700 rackmount chassis. Using high-quality M3 brass standoffs: Most NAS-specific cases (Synology