Probably because he didn't want to cut the drywall to run it through the studs and didn't have THHN in the truck...or he's an idiot. Of course, those two choices are not mutually exclusive.
If the new electrician is going to change it over to THHN, he could use 6AWG, but if he's willing to use 4AWG and wrestle it into the wall connector, that's great.
That said, I don't think the wire choice explains the tripping breaker. I have to think the OP either got a dud of breaker, or the connections at the breaker were not tight enough. Given the quality of the work, I'd wonder whether the guy just chucked some used breaker he happened to have in the truck in there.