There may or may not be a difference with a 2NL fish and a 400NL fish. That being said:
Fish calls preflop with any Ax, any pair (usually below QQ, monsters are predominantly raised), suited gappers and all broadways.
Fish continues with any pair, ANY straightdraw, ANY flushdraw (maybe not backdoor) and better. He probably didn't hit his draw, since he didn't lead a single street.
Then again, our Hero bet every street except river, so the fish might also have hit gin on the flop or turn and was looking to CR the river. So, the surprise might have been seing a set. My guess is 4♦:4♠: