iPhone 4S OSX 7.1.2 Wifi switch greyed-out (disable)
If the Wifi switch is being displayed as greyed out that means that there is a hardware problem with the Wifi modem in the device. I had faced this with my device and Apple replaced it under warranty, I would suggest take the device to an Apple store and they should replace the device as you get 3 months support on replaced devices even if it was a paid replacement.

P.S: Rebooting, Resetting or any other changes can not fix this issue as it is a hardware issue, the earlier you go to the Apple support the better.