Is there something about the fourth issue of a team book that means you have to (re) introduce a new character? Let's consider ...
The fourth issue is probably a time when you start adding in new stuff. The hero has got an origin and done a little bit of stuff and had any main characters settle in. So then new stuff starts getting added on to keep readers interested and add new storylines. There are probably new character intros for issue #3 and #5-6, too for various books.
It's mostly the trend of JLA-Avengers-Teen Titans that we're extrapolating farther than is probably reasonable. Still, that was probably the three major teams in the SA, so it's a noteworthy coincidence at least.