I like your response - you think!
No one is an unknown, you've always got some information on them. For starts, the pure fact the guy has taken this line means he is definitely not a reg, so you can discount that, and it's pretty obvious he's a fish. If you've got 4 hands on him then he must have posted to play too
My basic rule for fish is that they don't bluff. He's taking a ridic strong line for a fish (raising & pot betting). If you were struggling to fold flop, which is understandable, I think the turn is an easy fold without more information. You might find out he's a crazy maniac and you could have stacked him because he was bluffing, but that happens. There are definitely more passive fish at 2NL than there are aggro ones.
It was definitely worth iso'ing for value BTW.
Pascal, I really appreciate the question you're asking because it is an important educational point for me. Thinking about my own hand is that first level of thinking. I have to advance to thinking about what hands the villain has. Maybe I don't need to do more than that at 2NL (as some have suggested) but I have to do at least that. I also have to look more at the overall game they're playing in the hand and not just the action they're making on a specific street.
Always do as much as you can. The best bit about 2NL isn't the money - people don't play to earn 50c an hour. It's the fact you can plug leaks and work on your game, try out plays, make mistakes, etc, without losing too much money. This lets you improve your game, but it's a great place to figure stuff out.
Obviously you want to be moving up as fast as possible (to earn more money & because rake becomes less of a relative %) but don't just play 2NL to earn a few bucks so you can tackle 5NL. Work on hand reading in particular, and what hands someone is representing, as most 2NL players are very see through. Find out what happens when you raise/check raise/donk bet into different players, and see if you can then use that knowledge of how they react to your advantage.
Poker is all about adapting - if you knew exactly how someone played (e.g. they raise 100% of donk bets, they c/r 100% of low dry flops vs a c-bet heads up, they stab 100% of the time in uncontested multiway flops on the river), then you could always make money against them by taking the optimal line. Obviously poker is a game of incomplete information but if you are 75% sure that some raises donkbets with complete air then you can earn a lot of money just 3betting the hell out of them. And the best bit is, most of them won't even adapt back
Oh, and something I was told at 2NL which really helped my game and is worth thinking about - always think about what they are representing, and what you are representing.