I had to spend a lot more time reminding the 4's not to throw dirt, and to watch the batter, but other than that, it was fine.
Another reason (already raised) to be vigilant in how I do this.
What I finally decided to do:
Cutoff date has been moved to Apr 30 for all divisions with the exception of the youngest girls. Their cutoff is now Jul 31 - they must turn 5 on or before that date. This is in line with our previous rule, and with other local leagues.