Have you previously communicated with your daughter about good touch, bad touch, her private area, etc.
This is something I would think and hope you've been communicating with her for quite some time. If not, start now.
As for what happened at preschool, I don't see a 5 yr old as a molester and if you make a huge deal of this then you are setting a standard that touch is dirty, wrong, etc that will stay with her for years to come.
You need to carefully communicate that her privates are hers only and for no one to touch. This is a normal age for children to be inquisitive and I see it as the 5 yr old who touched her did not have proper communication at home either.
I would not blow this out of proportion because the last thing you want is for your child or another child to feel shamed and dirty.
Communication is key. It should start at a very early age and grow with the child.
Let the teacher know if you feel so inclined now that it's the end of the school year but please don't blow this out of proportion. The 5 yr old who touched her is more than likely as innocent as your daughter.
Teach your daughter boundaries.