Hi Thomas!
This is because we failed to include a database upgrade script in the RC4 release. This has been fixed, i.e. upgrading RC3 -> RC4 will work OK now, but those who upgraded when the script was missing need to manually run the script on the database.
You can obtain the script from our support by mailing to episerver5@support.ep.se and requesting the script 5_1_358_4to5_1_422_1.sql.