For the first 2 years of the play-offs I think it was possible to only have 3 promoted from the 4th tier as in the play-offs there were 3 teams from the 4th tier along with 1 team from the 3rd tier (last relegation place). If that team won the play-offs then only those automatically promoted went up, and only 3 went down. I don't think that happened though, although it was possible.
I think I am right, maybe UTM would confirm?