[Ryno]

First of all, I did not fold, though I thought about it.

Let's say that the only hands he could have are AA, A2, and 22. He would only play AA this way 10% of the time preflop, but A2 and 22 he would always call with a 3rd person in.

By the river there are 6 ways he could have A2, 3 ways he could have AA, and 1 way he could have 22.

I think this guy would 3bet my river raise with A2 50% of the time, and every time with AA and 22.

{Note I didn't really error check this but I think it's right}

AA: 10% * 3 * 100% = 0.3
A2: 100% * 6 * 50% = 3.0
22: 100% * 1 * 100% = 1.0

I have the best hand 3/4.3 = 70% of the time so I should 4-bet.

Now he 5-bets. How often does he need to do this with A2 for me to call? It turns out the answer is 1.1% of the time:

AA: 10% * 3 * 100% = 0.3
A2: 100% * 6 * 1.1% = 0.066
22: 100% * 1 * 100% = 1.0

0.066/1.366 = 4.8% chance of winning.

So a fold would have been pretty terrible.

He had 22.