Hi. WiFi on my iPhone 4s also got greyed out after updating to iOS 8.4. (I updated it yesterday to iOS 8.4.1 but nothing changed). The thing is, I went to certificated Apple service and they've replaced WiFI chip. Everything was working fine for about a week or so and then WiFi got dimmed again. So I send it back and they've replaced it again, since I got service warranty, but the problem came back. I know there is a big chance that motherboard is faulty but the thing is everything works fine except WiFi. Off course battery is draining fast after the update but phone is working fine. BEfore update battery could last 2 days, now 8 hours barely.
So my question is, did someone on this forum tried to replace battery and this way fixed broken WIFI. I read somewhere on some blog bellow at comments, way back when iOS 8 was released, that this actually worked and by replacing the old with new battery WiFi was alive again. Please share some info regarding the battery.