there are MANY myths about 4:20, I personally believe this one:
some kids at some school in sacramento or some other place in cali' used to meet up every day after school to get high. There school got out at four o'clock or so, and they'd always end up sparking up at 4:20. so, when they were at school and planning a smoking session for afterwards, they'd refer to it as "4:20" just because that's when they'd meet up. Anyway, somehow or another (i've read the greatful dead started using the term in flyers, but who knows) it "4:20" became the national trend it is today. just could've just as well been 3:50