Assuming you haven’t made any weird engine changes that modified the base object classes, migrating assets from a UE4 project to a UE5 project is as simple as right-click->Migrate in the UE4 project.
Going from 5 to 4 is impossible because once the asset has been saved in 5 it gets a whole bunch of extra data that 4 doesn’t know what to do with so it will either throw the ‘this asset was saved with a newer version’ error, or it will decide the asset is corrupt.