The iPhone 4 and 4S had a temperature sensor that would fail easily (usually in a couple of years, so outside your standard warranty) and would lead to the "wifi grayed out" issue (google it, the thousands of posts from that time should still be there, along with many iphone 4/4s listed on ebay etc with a non working wifi) - since the wireless module was disabled (taking bluetooth and gps with it). The official response from Apple was "reset your network settings", while users found that temperature shock (phone in freezer, then blow-drier etc) would "fix" it for a while. I keep a phone from each generation for testing purposes, my original 4 had failed that way, it was out of warranty and I replaced it with a 4S (fortunately company bought), which failed the same way and was replaced with a refurb unit, which, quite naturally also failed just outside warranty (all three phones were permanently on a desk and got a few hours of debugging/testing usage per month). What is infuriating, apart from the fact that Apple didn't care probably because their average customer was happy to just move on to iPhone 5 etc, is that it seems that it would be very easy to fix by software, ignoring the temperature sensor. I am not just saying it is easy, IIRC the iPhone 4 included the sensor without software support when it first came out, so whoever stayed on the original iOS version (was it 5?) did not have any issues regardless whether their sensor was working or not!
Let's see how my iPhone 6 plus does... It also gets very little use for some testing (I prefer Android as a personal phone), so the screen is fine so far.