Taking a stand PF with 99 against lag (I 4 bet)

[PleasureGuy69]

Villain is 48/23/6.8 and I'm pretty sure he's 3 bet me 2-3 times PF previous to this.

First of all, is this 4 bet ok?

Secondly, what do I do now on the flop? It's not great for me at all; however, his donk bet is extremely weird. If he had a big hand, he would have c/raised to get more money in right? Since I took control of the hand PF, it would be expected for me to bet on the flop and that's how he could get my money. By him donk betting, it seems like a huge bluff.

Anyways, critique plz

Full Tilt Poker - No Limit Hold'em Cash Game - $0.25/$0.50 Blinds - 6 Players - (LegoPoker HH Converter)

SB: $56.05
BB: $115.70
UTG: $46.15
MP: $53.25
CO: $47.50
Hero (BTN): $52.05

Preflop: Hero is dealt (6 Players)
UTG calls $0.50, MP folds, CO calls $0.50, <font color="red">Hero raises to $2.75</font>, SB folds, <font color="red">BB raises to $9.50</font>, 2 folds, <font color="red">Hero raises to $27.00</font>, BB calls $17.50

Flop: ($55.25) (2 Players)
<font color="red">BB bets $25.05</font>, Hero ???