I have not tried V3 to V4 yet but Morph Manager copied the morphs from an M2 outfit  I converted to an M3 one and most worked okay.  And I did a Hiro outfit for M3, the open morphs on that worked just fine when transferred. Morph Manager also let me copy Mike's body morphs into an outfit I'd bought that didn't have any at all. Now it's more useful, since it fits a bulky barbarian as well as a skinny elf. :-) 

If there are body parts one item has that the other doesn't, that might be problematic though.