UTG opens 2.5bb (50bb stacks), Hero 3-bets 8.5bb, UTG 4-bets to 19bb

Multi-way Preflop | 3-way: BTN | 50bb effective | Difficulty: Tough

Players in hand: UTG, Hero (BTN), BB

Hero hand

A♠K♣

Board

(preflop)

Action so far:
UTG opens 2.5bb (50bb stacks), Hero 3-bets 8.5bb, UTG 4-bets to 19bb

Pot: 11.5bb   To call: 10.5bb

Villain range: UTG 4-bet at 50bb: KK+, AK, occasional A5s bluff

Your move: