You can skip the back up. Click on the X in the status window. It will then start syncing.
However, do not do this if you haven't backed up recently. if you plan to use a restore for your new iP4 then you will need the restore point.
I skip the back up a lot and it always syncs with the changes done on the phone side and the itunes side.
I don't think you cans skip the backup during the iOS 4 update.