Could a pilot do this completely on his own? Turn off the communication system, fly 4 hours into a different direction and then nosedive into the ocean?
Yes.
Egypt Air is an example of a pilot committing suicide, before doing so he pulled the circuit breakers for most systems including some for the flight data recorders (black boxes).
For safety reasons a pilot needs to be able to quickly isolate any system (even ones that are 'vital'). If the pilot wishes to kill everyone, he will, there is no point trying to design against that. However as the vast majority of pilots are frequently making changes to the automated systems idea of the world for the better of the plane.
At the moment on many forums there are a lot of comments from people saying we need to clip the wings, so to speak, of pilots, limit what they can do. These people have never sat on the left.
This seems like really good news for would-be plane hijackers.
What exactly is known for certain?
If I thought that was possible, I wouldn't fly the little sheds I do for fun.
If not there, I would guess a landing in a dry lake bed in Tibet is next most likely.
But since there has been no communication, it raises the odds it crashed, either at sea or attempting to land.
The circuit breakers for the ACARS and radar transponder are accessible.