I don't think so, honestly I can't remember though and am trying to think myself what i've done in the past (bought every iPhone since the first one) I think the restore from backup doesn't downgrade the OS, just restore user created files and settings and stuff. Oh well, OS 4 on iPhone 4 to OS 5 on my iPhone 4S, will restore as soon as I can activate ( ). I'll post back an update if I beat you to it.