Can I use a library from an earlier version of rekordbox for iOS with rekordbox for iOS (ver. 4)?

Libraries from earlier versions of rekordbox for iOS can't be used with rekordbox for iOS (ver. 4). If you're using a PC/Mac, you can convert the library from a version of rekordbox for Mac/Windows earlier than ver. 6 to use with rekordbox for Mac/Windows (ver. 6). Then you can sync it with rekordbox for iOS (ver. 4).