if you want just v4 for her expressions but like the body of v3 here is what i would do instead on trying to weld stuff together that probably will not work...

just place you v3 where you want to..then render her..then place v4 at same place and pose and make sure that neck and head are roughly the same size also try to have a skin for v4 that matches your v3 closely foe the skin saturation......delete v3 and render v4...

go into photoshop and open v3..open v4 then add it on top of you v3 as a new layers and erase the part of v4 you do not like leaving only the stuff you want to see from the 2 plates...merge the layers and clean up any non matching artifacts.

If you are just doing one image i think that is what i would do...i did not try this myself...god knows why but i think i would do this...

i do this all the time if i want to add stuff that i am missing from previous render or want to properly had clothes because of boobs size problems and such...

this probably will not help much but it is an idea....

cheers!