Actually, the popularization was made by the Grateful Dead's use of the reference; "Is it 4:20 yet"?
But, according to one source:
..." Quite simply, the birth of 420 occurred at precisely 4:20 in the afternoon to begin a bedroom bong session at the house of Du and Puff on a Saturday in October of 1970. The Bebe along with the brothers began preparing to “bong out”, when Bebe glanced at the clock on the nightstand and said, “It’s 4:20, time for bong loads”. After getting high, they proceeded to do some audio recording with Bebe, as we did frequently, using his assortment of voices, including his impression of Abraham Lincoln, and said as tape was rolling: “4 score and 20 years ago…” -420 Magazine
