Its definitely not too late age wise! I've see lots of adults competing xcel. She would have to compete silver and then score out to gold/platinum. Timing wise, I would contact teams and look for an eval pretty quickly. Although girls at my DD's gym sometimes join team in March/April, but they...
I agree that you should have a meeting with your coach. The coach probably wants her at L3 for better competition results. However, if she feels like she wants to push herself to L4, then talk to the coach about it, and make sure that her feelings are heard. Good luck!
My DD is a rising freshman in HS, and is an Xcel Platinum. She knows that is is very unrealistic for her to do gymnastics in college, as there is no way she could get to Lvl 9/10 in 4 years. But she wants to be competing in the sport after high school, what are her options?