(click to enlarge)

What BastBlack and Cage said.

Attached pic shows a "V3 to V4" hybrid.
The advantage is that all V3 headmorphs work 100% right out of the box. Just inject your V3 character and that's it.
The disadvantage is that you need V4 textures for the body and V3 textures for the head, so you have to find two textures of matching color. (Or match the textures yourself in Photoshop before you apply them.)

I successfully used Cage and Spanki's Phyton script both to create my reduced resolution  MIKI and Sydney meshes, as well as to transfer V4's expression morphs over to V3RR.
:thumbupboth:
Worked like a charme, but as the process is still a WIP, be prepared to do some manual cleaning up of the morphs.
Especially V4's causes a lot of problems because all those gimmickry DAZ put into her cr2..

The advantage is of course that only the shape is transferred, not the actual mesh, so if you use that script all original head morphs will still work and of course the mapping stays the same, too.
(So no need for "patchwork" mapping.)