https://www.gobilda.com/m4-x-0-7mm-nylock-nut/?sku=2812-0004-0007-250pk

These M4 locknuts have a nylon insert that the threads of a mating screw must form threads in upon insertion.  The displaced nylon will continue to hold tension on the screw threads which act as a locking mechanism to prevent the screw from loosening under vibration.