A few things that might be interesting -
The early RV-3 engine mounts only had the four perimeter attachment points to the fuselage.
The current RV-3 engine mounts have two additional ones in the bottom middle. These are co-planar with the others, since the RV-3 firewall is flat.
The current RV-4 engine mount has six mounts somewhat like the RV-3 mount, but the middle bottom two are inclined forward of the vertical and are not in the plane of the others, because the RV-4 has a different firewall that includes an angled bump-forward to allow more foot room. I don't know if older RV-4 engine mounts also had these two additional points.