[iggymcfly]

4-betting against unknowns is beyond stupid here, you could easily end up isolating with UTG's AAxx which is the last thing you want to do with this hand. Either call or fold.

Also, while you don't have a huge piece of the flop, you are in a pretty good bluffing situation. I think CO likely has a high rundown hand and even if he has AA, you might get a fold. I'd probably check/min-raise the flop and then push the turn as long as UTG gets out of the way. This would be a better play if you were just a little deeper however.