4(20 is something that originated from a pretty long while ago but it is now basically slang for weed/smoking weed.

I quote Urbandictionary:
The term 420 originated at San Rafael High School, in 1971, among a group of about a dozen pot-smoking wiseacres who called themselves the Waldos, who are now pushing 50. The term was shorthand for the time of day the group would meet, at the campus statue of Louis Pasteur, to smoke pot. Intent on developing their own discreet language, they made 420 code for a time to get high, and its use spread among members of an entire generation.[/b]
We had heard rumors that one of the members brothers had planted a pot patch in the wooded area around town. We all wanted to go look for this patch of weed, so we devised to meet eachother by the statue of Louis Pasteur after school. Since we all had after school activities or electives that lasted about fifty minutes, and since it took about ten minutes to get from school to our after school activities, and another ten minutes to walk from the activites to the statue. Since there were many of us we decided that 420 would be a good time to meet so we could go search for the patch of pot. We never did find it, but we had lots of fun getting high while trying to look for it.[/b]
Edit
Hey sloth, this is coming from the guy you called an asshole and I'm the only one not calling you a stat whore. Oh, the irony!