You can use a 4 pin PSU cable in an 8 pin CPU power slot, but it depends on the CPU whether you want to keep it that way or replace the PSU to get the 8 pin connector. An intel i3 or ryzen r3 will be fine with the 4 pin from the PSU, but you may have issues with an i7, i9 or r7, r9 CPU.
As...