AK btn vs blinds 4 bet pot

I've switched from FR to 6 max recently and AK is a hand I'm having a little trouble with. 3bets and 4 bets don't get quite the respect they did in FR which puts me in a few more spots post flop...

Anyway, sb is very loose and aggressive, he's 3 bet me several times 60/30/11.8 3bet over 50 hands, agg factor of 47. BB is 26/17/6.7 and seems fairly straight forward, I'd expect him to want to play a lot of hands vs sb if he's paying attention. I had not 4 bet sb yet and I'm playing about 26/21 or so on this table.

So is my 4bet a little small? Maybe with the caller it should be bigger. Now with them both calling a 4 bet I'm more worried about one of them having a monster, so just c/fold? Or if you bet and try to take the pot, how much?

PokerStars No-Limit Hold'em, $0.50 BB (6 handed) - Poker-Stars Replayer from Poker Hands Replayer

http://www.pokerhandsreplayer.com/po...ctures/742.gif

UTG ($2.25)
MP ($53.15)
CO ($49.70)
Hero (Button) ($50)
SB ($118.75)
BB ($58.80)

Preflop: Hero is Button with Ahttp://www.pokerhandsreplayer.com/poker-cards/spade.gif, Khttp://www.pokerhandsreplayer.com/po...ds/diamond.gif
3 folds, Hero bets $1.50, SB raises to $4, BB calls $3.50, Hero raises to $13, SB calls $9, BB calls $9

Flop: ($39) 7http://www.pokerhandsreplayer.com/poker-cards/heart.gif, Jhttp://www.pokerhandsreplayer.com/po...ds/diamond.gif, 4http://www.pokerhandsreplayer.com/poker-cards/spade.gif (3 players)
SB checks, BB checks, Hero ($37) ?

free card, fold if they bet and you whiff. the new thang is flattin' 4bets and shipping non A or K flops

I'd be 4xing it at 50nl. As played c/f unless you hit an ace or king on turn or river.

So is a 4bet here better than just calling the 3bet? And yeah 4x seems better.