Repeating a level is very common. At our gym, it was more common to repeat 6 than 4, but that's changed since we our former head coach left.
Our former head coach was someone with 20+ years of experience in gymnastics and a bunch of kids of her own (I want to say 5?). All her own girls did gymnastics. One stuck with it and competed NCAA. Anyway, she was very practical about the team. She knew ours was not the place to be for kids or parents with Olympic dreams. So, our team almost never had girls who barely made the minimum ages set by USAG because there was no rush to get anybody through the levels. She preferred the to take older girls for the simple reason that they were more mature, and she felt more comfortable that the girls, not the parents, were making the decision to be on team. I think this is why the only "repeats" we had at level 4 were the girls who just couldn't get a skill they needed for level 5 (usually the kip).
We see a lot of repeats at 6 because it's just a tough level. The skills get bigger, the scoring gets tougher, and the competition gets tougher as girls drop out. Getting to the podium takes more work.
At the optional level, we start to see repeats because the next big skill eludes a girl. We might also see a girl repeat a level rather than be the "only" level whatever on the team. It's something my dd2 is facing. If she stays in and goes to level 8 next year, she'll be the only level 8. Not much fun. I suspect she will retire rather than repeat 7. I know she won't want to compete alone.
Anyway, back when we were just starting this journey, some of the experienced moms told me just to expect to repeat a level at some point.
To answer your question, though, my girls each did 1 year at level 4. The oldest was 10 when she competed L4; the youngest was 8.