Sorry, I just noticed you said you fixed the issue with the mesh..

I don't have V4, so I can't really give you a specific fix on the rig, but I would look at the joint centers on the above mentioned parts.

Also the hybrid might be causing a problem. Perhaps give it a standard rig and use the morph brush to make the abdomen do what you want it to do?

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.