Unfortunately not. Many of the sound-engines in AL4 are incompatible with AL5 because Arturia re-coded them. If your projects use sounds exclusive to AL4, you will need to carry on using AL4.
EDIT: And just to pre-empt someone saying 'it sucks that AL5 is not fully backwards-compatible with AL4' (I've seen this complaint various times), no it doesn't. In order for a developer to enhance a product without dragging-along legacy code for a hundred years or more, there has to come a point when they draw a line in the sand and decide the new version of something isn't going to be 100% compatible with the old version, otherwise progress can't be made. It would only become an issue if the new version took away something you legitimately purchased. And seeing they let AL4 purchasers keep AL4 as well as AL5, there is no issue.