Maybe I'm wrong (but I don't think so), but as far as I can remember the playoffs have always, always, always been 4 our of 7 for every round for the last 80 years, at least. No exceptions. I never recall any 3 out of 5.Mostly because the first 2 rounds were 3 out of 5 and not 4 out of 7.
As I'd posted earlier they used to play the 1st 4 games in 5 days. That is one reason they finished much earlier.