iPhone 4 wifi stuck on off
There is nothing that can fix this other than hardware service, it's greyed out because the Wi-Fi module is not functioning correctly, it is intermittent now but will eventually fail entirely.

Your best course of action if you are under warranty is to take it to Apple, however this model is unlikely to be under the 1 year warranty, or 2 year AppleCare warranty.

In this case, if all of these apply to you:


Bought the phone in the EU.
Are the original purchaser of the unit.
Bought for consumer use, not business
Bought from Apple Online or an Apple Store


Then you can make a claim under EU Consumer Law which states a device should last a reasonable amount of time, however you are typically required to demonstrate this within the first 6 months. However this issue was a common failure mode for this product so I would suggest that it's a known issue to Apple and they should replace it under EU Consumer Law on the basis it is a known issue.

If one or more of the above don't apply or Apple refuses, then there is no option but a handset replacement, I wouldn't personally attempt a logic board replacement using parts online, it's a particularly complex repair.

Source: 6+ years working at Apple Retail