Hi!
Okay, first things first! Please open it up and clean it completely, remove all the dust you see. After this, replace the thermalpaste and then assemble it again.
Guide for replacing the thermal paste (and for disassemble): [guide|135635|PlayStation 4 Slim Thermal Paste Replacement - iFixit Repair Guide|new_window=true]
If this didn't solved it, you may want to check the power supply. But let's try this first, the PS4 should run way cooler then before!
Please let us know how it goes!