My experience would be it's OK to wait. My daughter did well at Level 4, too--scoring 36s with 2nd place AA at her best meet. Based on that, we all thought she would rock Level 5, and she was so proud to move up. However, it really is a different level of competition, expectation, and scoring. At Level 5 she took home only a couple individual event medals all season (no AA places). Our daughter "had" all of her Level 5 skills, but the judges don't score on whether or not they "have" the skill, they score on whether or not the skill is perfect. It's not easy consoling your daughter because she didn't get a medal, and it's hard on her self-esteem to sit out so many times. We certainly focused on "personal bests," improvement and growth, but what a little gymnast really wants is a medal! An extra year at Level 4 will give your daughter time to get stronger and more powerful for the vault, hold her handstands longer on beam, achieve perfect form on her handsprings, etc.
Our daughter now "has" her Level 6 skills. But if she stays at Level 5 for a second season, she'll probably have a much more positive/rewarding experience than if she moves up without having "perfected" her new skills. (For her sake, I'm actually hoping she stays at Level 5)! But either way, it's not my decision, it's the coaches'.
If your gym has a strong team at each level, then they must be doing something right as far as uptraining, and I wouldn't worry too much about her not moving up right now. If not, then that is another question . . .