Often known as a B.B.B.B. or B4, it's the act of sending a close-up picture of a penis outline, visible through a pair of boxer briefs. The B.B.B.B. is most often sent via a social networking site.
Did you hear about the Boxer Brief Boner Broadcast from Congressman Andrew Weiner? He assumed that ladies wanted to see his penis outline through his boxer briefs. This is one B4 he will surely regret.