"I would imagine that you could inject V4 morphs into the head, I haven't tried this."

DAZ' "V4toV3" can't use regular V4's headmorphs either.

"V4toV3" has her own set of expression and head morphs, but if you buy a custom made character for regular V4, that new headmorph can't be transferred over to V4toV3.
(Unless it's just a combination of stock DAZ morphs.)

So she even is useless in that regard, unless a merchant also adds a "V4toV3" friendly head morph to his character pack.
(Like recently happened with the "Emma" morph)

Of course you can combine (standard) V4's head with regular V4's body yourself.
Such a figure could use all V3 bodymorphs and all V4 headmorphs.

The problem is, unless someone remaps V4's head (Or V3's body), you have to use two different sets of textures:
One for the head, one for the body.

Not impossible, but definitely a nuisance and most likely too difficult for beginners.

You also run into trouble if you want to distribute such a figure, because it uses both V3 and V4 mesh.