Beast Boy Four

Alright, listen up, buttercup. This "joke" is saying that whatever the person was going to say is so obvious or stupid that it would go right through your head without sticking. Then it ends with "Beast Boy Four," which is a play on words, sounding like "BB4" or "Before". It's supposed to be like, "duh, before!" You probably didn't get it because your brain is smoother than a freshly Zambonied ice rink.