Evilmastermind

If the 2/4 is full of regulars you should maybe never go there, unless there is a total drunken whale their that is just gifting money away. The game isn't much larger at all to 1/3 and when the game is clearly very much harder to beat, then you are just losing money compared to 1/3. You can make more money there for sure.

And otherwise 20BI is way too little anyways to take on the 400NL. You should stick to the lower games in that regard asswell atleast when you have 50BI's and that even is pretty liberal.

And if you have palyed only 6 weeks you need way more expirience (years) to take on the good players to have an edge.

I think as hard it is, it's a clear fold. You can't beat anything than a bluff, and it would be totally and absolutely insane to just call on the flop to bluff his whole stack on turn and river, because there is no drw on the flop. If he just won a big pot then he has no motivation to go crazy, especially when people saw how bad hand he had, so he would have no credit to do that. Even the bad players know that they cant credible make a bluff just after that hand. And like i said it would be insane anyways. Easy hand when you think of it through.

q4: could offcourse have set if he is a scared-easily player who scares that the flush will come, so get some reads on the players.

First of all you need to give more information of the players and how they play. Usually it's impossible to answer to hands questions without them. Poker is more about the players, less about the cards. Especially the question 2 is totally about how the all in player plays.

otherwise i would go like this : Q1: easy call, could be bluff or betting easily worse for value
Q2: no idea, depending of opponent, to really tight players you should have folded preflop.
Q3: again about the player, and you should have folded preflop anyways. But would call if no reads, because good hands should generally just call flop.
Q4: Usually overbets when a flushdrw is on the board is a flushdrw with overs, so i put him on KQs QTs T8s and such so would do the same line as you.

Hopefully this helps.