Lowering the engine 4mm won't hurt anything, but you still must get the cowling to
align with the spinner. Usually there is some wiggle room when mounting the cowling,
and it will still fit OK.

The whole deal with the offset marked on the firewall, is because there is an angle
(engine thrust off-set) built into the firewall. You must shift the engine to the side a
little bit so the engine propshaft sticks out close to the center of the front of the cowl.

If you didn't shift the engine over, the propshaft would be too far over to the right.

FBD.