M4 Spring Washer (Square) in A2 Stainless - DIN 7980
Square Section Spring Washers are designed to help prevent loosening of bolted joints under vibration. Also know as Split Lock Washers and Split Ring Washers.
This type has a square section and is generally used with Socket Head Cap screws/bolts, manufactured to DIN 7980.
Where even distribution of load on the mating surface is the primary requirement an alternative spring washer or a flat washer would be a more suitable solution.
In addition Spring Washers do not offer protection for the material being fastened.