4 players:

UTG 100bb - only few hands with him, doesn't seem fishy
hero BTN 100bb with A 4


UTG raises 4bb, hero calls, 2 fold

flop (9,5bb):
4 2 K
UTG bets 7bb, hero raises to 18bb, UTG calls

turn (45,5bb):
5
UTG checks, hero??


- what is the best play here??


I thought about this play for last 3 hours, do the math, but I'm not sure