If the girls are good enough, I'm sure they can go on the 5th grade team. Just sign up for the tryouts. Worst case scenario, you paid for a two hour clinic and she knows better what to expect next year. Nothing to lose. And if enough girls did the tryouts, I'm sure that they would form a 4th grade team.
I'm not so sure they would form a forth grade team. All the tournaments, the lowest start for the pools is 6th grade. So a fifth grade team is already pushing it. But your right, forth graders can play up with the fifth graders. It's amazing the physical size difference of a fourth/fifth grade team playing against pure sixth grade teams.
Actually fifth grade is a little late to specialize in lacs. I've heard through a reliable source that most of the for profit organizations are going to begin teams starting at one year old. They've created a pacifier that doubles as a lacs stick for a small ball. The only requirement is the child needs to be able to take 4 steps without stopping and has to be potty trained so there is no number two going on during practice. Costs will be a little higher than the older girls programs but the early start is almost assurred to result in full college scholarships for all.