Probably the thermal compound on the heatspreader (I think that's the name of it) wearing off. You can temporarily fix that by opening it up and replacing it with some new compound.
if you can get it to turn on Take it to bestbuy they have a $150 voucher for trade in on PS4's this weeknd
The problem with sending it to sony is that it costs a couple hundred I hear and sometimes they'll just be like "we can't fix it sorry"