I totally understand where you are coming from. My DD recently moved to L4 and it was definitely a change for her. She loves it and hasn't complained, but I wonder sometimes. She doesn't go to school, so she hasn't been tired after practice.
We are lucky in that there are now about 6 young L4s and they group them together because of their skill level as well. 2 are 5, 3 are 6 and 1 is 7. They are together with the older girls for strength and conditioning but are usually by themselves for skills stuff. I think that eventually they will integrate them in with the other L4s and they are with them for some skills training like learning the floor routine.
I worry about it being too much for any little girl. I have struggled so much with the decision to let her move to team. I stayed the first week and now I am only there for about 1 hour of the practice. We have a new parents meeting and first booster club meeting next week, so I will stay until those start and actually see more of the practice again.
The mom of the other 5 year old asked the gym owner about some things and she said to give it a good 2 months and then see. She said it takes awhile to get used to the new schedule and extra conditioning, etc. We were more worried because they all seem to lack the attention span that the older girls have, but she said that is normal and the coaches know how old they were and that they eventually will get better at paying attention.
I think as long as you have good, open communication with your DD, things will be fine. Talk to her about how she is feeling, etc and remind her to tell you if she hurts or has any issues.
Hang in there. I think it is harder for us than it is for them!