Was the CO first in with that raise to $4?
Assuming he's solid, why would he play a set so fast? He's in position in a reraised pot and your CB was strong. He should be able to make a regular raise and count on getting all the money in by the turn.
The only hands that make sense are
[Kc] or another big draw of some kind, an overplayed QQ/TT, or AA pulling the Iceman power play.
If you're willing to go broke every time at 6-max AA vs KK (some people are; some aren't) then I would just discount that possibility, put him on a draw or a lower pair, and call.