Why should I 4-bet 99 here?

Game: WPT Pace 0.02/0.05/0.1 Pace 8-max

Hero: SB, 720BB
V: BB, 300BB

All folded to Hero.
Hero raised to 6BB with 99,
BB 3-bet to 18BB,
Straddle folded.
Hero's next move?????

A friend watched me play this hand and said that I should have 4-bet instead of calling in this situation. He said 4-betting generates more EV but did a poor job explaining to me why. I don't understand why I should reraise in this situation - playing a bloated pot OOP with a medium hand/turning it into a bluff. If it is truly the case that 4-betting is better, can someone provide me some explanation?