any idea ???
I would recommend downloading 6.1.1 full via iTunes, and then not restore the phone from backups until before you check to see if your WiFi has re-enabled itself after the restoration. If your WiFi works, and you then perform a restore from backup and it fails, you know it's your backup thats corrupting the phone then. I find often if you went from 5.1 to 6.x and then restored your backup from a 5.x dataset, it will often cause problems.
If it's determined the backup is causing the issue, you'll need to re-load everything onto your 4S from scratch and start new, and then make a fresh backup afterwards.