... Um... No, OS used to be a 1 year internship before WWII, then 2 years, then 3 years for a short while after, then there was a shift in the dental world saying that Dentists should have more medical training, and this ideology continued to the 1970's when OS became OMFS and required a minimum of 4 years and about a decade later many 6 year tracks started popping up.
Even before then, MD DDS was a thing for a long while tracing back all the way to the mid 1800's, because many oral surgeons went back to medical school to get their MD. James Edmund Garretson, the father of OS was a dual degree MD, DDS guy and he was from the 1850's.
So no, 4 years was not the standard for most of our specialty's history. I have no clue why anyone would assume that.
Source: Took an elective of history of dental profession in dental school, and did a project on the history of OMFS. Had a professor that graduated from a 3 year track waaaaaaaaay back.