Restore iCloud to an older iPhone, e.g. from 4S to 4?

My friend recently had his iPhone 4S stolen but had a somewhat current iCloud backup. He's trying to get a temporary phone as he wants to wait for the new hardware later in the year and has been offered another friend's hand-me-down 4. Can you restore an iCloud backup to older hardware? And does it need to be running the latest iOS before restoring? Any pitfalls to be aware of? Any danger that the backup gets messed up in the process? Thanks!