That Central plate is for a SMLE. Mounts using a longer trigger guard screw, and the safety spring screw. There is a hole for the safety pivot, and a coil spring would be used to tension the safety.
There is another version which could be used on either a SMLE or a No. 4. To mount on the No. 4, the trigger guard screw is replaced, and the safety spring screw position is used.
As Cantom pointed out, a hole would need to be drilled in the butt socket to use this plate on a No. 4. Not a good idea.