Droid 4

Do you change your phone to the same Motorolla Droid 4 model?  If same, maybe its time for upgrade to another make/model.  

How do you charge your phone?  Genuie Motorola charger?  
Maybe you have a tight case that blocks the air circullation.

So my suggestion would be to get a new and completely different phone.

Its not genuine wall charger.
Back cover is not very tight as its already broken from one side.

It only does that when I leave the wifi ON in the phone for hours.

You said you've changed the battery 5 times. Are you getting genuine replacement batteries? Non-genuine batteries can often be of poor quality and have issues like bulging and even start fires.

The Droid 4 is almost 5 years old at this point. Is there a particular reason you're trying to stick with this phone? Newer model phones will be faster and have better battery management, even on wifi.

Double check the spec of your charger to see whether it is of 5V / 0.85mAh.

Although there is a protection in the circuit board for over amperage protection, but your device is just too old and the board has been worn out already.

Stop using your current charger immediately.

@Jackie Man

I don't think "over amperage" is a thing. You might be confusing it with over voltage, which can damage electronics, and which some electronics compensate for to some degree.

Amperage is something "pulled" by the device. If you have a 1A charger, and the device only needs 0.85A/850mA (mAh is a rating for batteries, not chargers), the device will only pull 0.85A from the charger.

Think of a 20A circuit in your home for you 120V AC outlets. It doesn't mean 20A is running through it constantly, just that 20A is the maximum amount of Amperage that circuit can provide. If you hook too many devices into that circuit and start pulling more than 20A, the circuit breaker will trip, but the devices won't be damaged. Likewise, if there's only a single 1A device plugged into that circuit, it's not getting the full 20A, only 1A of the power that circuit can provide.

So a higher amperage charger doesn't do any harm to a device.

See this link for more info:
https://askleo.com/can_i_use_a_charger_that_provides_the_same_voltage_but_a_different_amperage/

Over amperage has an effect to make the battery going south faster.

Do you have a reference for "over amperage" being a real thing? Again, based on the references I can find, the Driod 4 isn't going to pull more amps than the device is designed to pull. So even if you plug it into a 2.4A charger, it's sill going to only pull 850mA, if that's what it's maximum amperage is.

More supporting evidence:
https://www.extremetech.com/computing/115251-how-usb-charging-works-or-how-to-avoid-blowing-up-your-smartphone

If you take a phone which came with a 900mA wall charger, and plug it into a 2,100mA iPad charger, as an example, will it blow up?

In short, no: You can plug any USB device into any USB cable and into any USB port, and nothing will explode — and in fact, using a more powerful charger should speed up battery charging. We now do this all the time with our mobile devices here at ExtremeTech, and we’ve never had a problem.

http://www.makeuseof.com/tag/why-cant-i-charge-all-gadgets-from-the-same-charger-makeuseof-explains/

However, when that rating shows an amperage that higher than the phone’s rating, nothing is going to go wrong. This is because the device “draws out” the amps from the charger that it needs – the charger will not pump the maximum rated amperage into that phone.

@Jackie Man the link you provided is a discussion on the "Droid RAZR M," which is a different phone. However, several of the posts within still support the idea that overamperage isn't a problem:

The charging circuit is in the phone, not the charger. The rating for the charger can be 2.1A but the phone is only going to take enough current as allowed by the phone. For a lower rated charger, you get a slower charge. Above 850mA you don't see a benefit. Your wall socket is rated at 15A but your PC draws 7A, its similar with the phone/charger.

Yes, you can have problems with different chargers, especially cheap ones on certain devices. However, I suspect such problems would be more likely an effect of overvoltage possibly, or even undervoltage, with the charger not outputting a clean 5V, and the device in question not able to compensate. Again, overamperage can't really be a thing since the device itself is responsible for drawing amperage from a circuit, so there's no way for a charger to deliver more amperage than what the device is pulling.

Also, I'm not the author, but a fellow expert disagreeing with your assertion.

My phone is old and yes the wall charger and the battery are not original. I bought online from ebay and pay under 10 bucks for both of them.

It's the cheap charger and the non-genuine batteries that are the problem. Cheap batteries are known to often be inferior and can bulge.

See these articles about the problems with buying cheap, generic batteries:
http://www.howtogeek.com/172680/danger-why-you-shouldnt-buy-cheap-third-party-batteries-for-laptops-or-smartphones/
http://www.jegandsons.com/why-you-shouldnt-buy-a-generic-smartphone-battery/
http://www.ebay.com/gds/How-to-Choose-a-Replacement-Battery-for-Your-Cell-Phone-/10000000177629503/g.html

The phone is so old it may be hard to find genuine batteries. Instead of eBay, I would look to Amazon to find a battery with some positive reviews on it. This listing seems to be genuine batteries, although pretty old (one reviewer got one 22 months old):
https://smile.amazon.com/gp/product/B00MFARH7C/

You may also want to think about getting a better charger. It doesn't have to be a Motorola brand one, and the amperage can be higher, but a quality charger can help. Cheap chargers cut corners that can lead to "dirty" power that can damage batteries and possibly even start fires:
http://www.geek.com/apple/why-cheap-phone-chargers-cause-fires-1593785/

Does any one knows about droid 5? When it will be available in the market.

Love QWERT. Just don't know why Motorola don't want to launch droid 5.