3/6NL: KK in 4-bet pot facing turn push

[Proofrock]

Villain is 19/17/11 after 306 hands. I've been playing pretty tight and haven't gotten out of line. This is the first time in recent memory that I'd 3bet him, and definitely the first time he 4bet me preflop. I'm having a hard time believing I'm ahead -- the only hand I can think of that I might be ahead of is AhKx. Thoughts?

PokerStars No-Limit Hold'em, $6 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

saw flop|<font color="#C00000">saw showdown</font>

SB ($694.70)
BB ($156)
UTG ($609)
MP ($1459.45)
CO ($600)
Hero ($1642.20)

Preflop: Hero is Button with K[img]/images/graemlins/heart.gif[/img], K[img]/images/graemlins/club.gif[/img].
<font color="#666666">2 folds</font>, <font color="#CC3333">CO raises to $24</font>, <font color="#CC3333">Hero raises to $72</font>, <font color="#666666">2 folds</font>, <font color="#CC3333">CO raises to $172</font>, Hero calls $100.

Flop: ($353) T[img]/images/graemlins/spade.gif[/img], Q[img]/images/graemlins/heart.gif[/img], 8[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
CO checks, Hero checks.

Turn: ($353) T[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">CO bets $428 (All-In)</font>, Hero folds.

Final Pot: $781