I def think you are ahead of his range here. Its kinda of a tricky spot cause you dont want to just throw it all in there bein 2 stacks deep. However, there is alot of draws he coudl be semi-bluffing with so just calling the turn for pot control could be dangerous.
Hands that beat you: 4x, 44, 99, TT, KK, AA. That is a pretty narrow range of hands. AA, KK you would expect a 3bet PF of course as well as TT or 99 would be 3-bettable. Particularly since he is a 21/23. His most likely bb Cold calling range would be suited connectors and small pairs.
I think his most likely hand is a diamond and/or str8 draw that he hit with his Suited connector or TP+FD ([Td]
).
Best lines are:
**Raise to 6k like Al suggested
**Or just call and reevalute the river. If its a blank and he fires again I might call. If its a diamond and he fires again I probably find the fold button. However he could def bluff the diamond of course with somethin like a missed straight draw. I think he will actually give up on the river alot of the time if he misses.
I think i would call the turn and actually FOLD alot of rivers if he fires strong again.