Part: Part Five - Betting Before the Flop

Facing a four-bet with A-K

Pre-flop
Blinds 100 / 200Pot 5,500 (27.5 BB)BTNA♥K♠● 20,000 (100 BB)YOUyou 3-bet to 1,800CO● 20,000 (100 BB)4-bettor4-bets 5,200 (26 BB)SB● 20,000 (100 BB)posts 100 (0.5 BB)BB● 20,000 (100 BB)posts 200 (1 BB)D

A player opens the cutoff, you three-bet the button to 1,800 with A♥K♠, and he four-bets to 5,200. You're 100 BB deep against an aggressive opponent.

He four-bets to 5,200 (26 BB). With A-K, 100 BB deep vs an aggressive four-bettor, best?

WhyFive-bet shove. Against an aggressive four-betting range - which includes A-Q, worse, and bluffs - A-K has strong equity and big fold equity. Shoving lets you win it now or flip/dominate when called, and avoids playing a bloated pot guessing post-flop. Flatting plays poorly, and folding A-K to a wide four-bettor is far too weak.
What happensYou shove; he folds A-Q face up.  You win a big pot pre-flop.
Against an aggressive four-bettor, A-K is too strong to fold and plays best as a five-bet shove - you fold out his bluffs and have equity when called. Stack depth and opponent tendencies set the answer.

A-K facing a four-bet is usually a get-it-in hand against aggressive players - shove for fold equity plus equity rather than fold or flat.