Pro as long as the user authentication etc is the same will need not much of any rewrite for vb4 maybe a little work here or there on the editor etc.
As far as vbGallery yes that will need alot of changes most likely since it interacts alot with and uses alot of the vb
However unlike Invisionboard where I got a copy to work with on vb4 we will have to wait till we get access to it to work with it. I can not say when that will be. They expect an alpha release sometime in august which is full but I applied for the beta but with no response yet.