Quote - Anything made for P4 can be used in P4 and all later versions.
Anything made for PP can be used in PP and all later versions (but NOT in P4).

Actually, you can use anything made for PP in P4, you just won't see the thumbnails in your libraries without using an PNG to RSR converter. As far as I know, PP through P7 will convert RSR's made for P4 to PNG's automatically.