On the L4 front end the top A-arm mount has a slot in it where the cross brace goes. Then the hinge pin holds the cross brace in.
On the L3 front end the cross brace couldn't be used because it would cause the front end to tweak. I seems the L4 has the same problem though.
Just run either one with out the cross they are fine. I do like the L4 blocks better and you get a 5* block where the L3 only has 0* or 10*.
take care
john