There was a story on 20/20 or some such show that the term began because it represented the time the kids at San Rafael Hign got out of detention and would plan to meet after detention.
Thats a good question. One thing I have heard is when the school bus from Tam High arrived in Bolinas it was around 4:20 , and the kids would cut loose with all the good boast on the beach.
First and foremost is the term " BOAST " started by Tam High kids in the early 70's. That meant : to roast or smoke marijuana. Controlled burn : the burning of a controlled substance. Puli Paca : original Hawaiian meaning : funny smoke. Larkspur Logger : a fat joint rolled in Larkspur. Pot Sticks : What people in Tiburon called marijuana cigarettes. Toots and Thai : snorting peruvian flake and smoking thai pot. Spreckled tabs : Pink tablets with blue spreckles in them that were 1000 micrograms of pure Sandoz LSD. Orange Sunshine : a drug that was a 3rd cousin twice removed of LSD , called ALD-52, it was not illegal until a few years after LSD was. It was crappola ! Bo York : Bolinas after the N.y. invasion. Mill Hatten : ditto. Sesh Posish : A position where we would have a marijuana smoking session. They were numbered so if someone said meet at posish #2, only those who were in the know would be there.