55 river bluf

It depends on his hand. He has a lot of hands in his range that miss the board and the fold is completely justified

He cbets in 4handed and you dont't close action. Imho is a fold flop, as played is speawy

I am not sure how it works on NL20 but trying to make people fold top pairs, especially Aces, is not a good idea imho. I would probably fold flop because he has a tight UTG range and continues with an aggressive play 4way entirely out of position on an A high board. That is enough for me to just fold right away. The additional gutshot equity is not enough.

Well, the problem is that we are like 10 regs on this limit and we have over 3000 hands between us and i think they don't expect me bluff the river too much. And I don't expect him to check the turn with a hand with which he wants to gain value

Its a fold on the flop imo, certainly with another player still to act.

villain mostly has Ax after the flop cbet into 3, and always has at least Ax after the turn call.
AQ might have improved but, for the most part, most villains are capped. it mostly comes down to whether he's generally a "believer" type or not, your range is certainly strong here.