You can't restore a device to a backup made on a device that was running a more recent version of iOS. If your iPhone 4 was running iOS 6.1 when you backed it up, you will have to update your 4S to iOS 6.1.1 (the latest version for the 4S) before you can restore it to this backup. After updating your phone, go to Settings>General>Reset and tap Erase All Content and Settings. Then go back through the setup screens again and when given the option, choose Restore from iCloud Backup. You should then be able to select your backup to restore to.