The Waldos interview (Lagunitas, 2014). There are lots of purported origins of 4:20 (usually about as well-explained as "oh, I heard that some people in some place..." or "I heard there was a police code.."
Not that I've put much research into it, but this seems like the most believable I've seen.
https://www.youtube.com/watch?v=_hIC7F-qe9Q