I'd second the motion to talk her out of it until she's a little older.
Turtles can be great pets, but they are not the easiest animal to take care of ..correctly. Water quality is a big deal, as is food, and UV.
She will no doubt want to handle the turtles, and CTChin is right on the money with the salmonella reminder.
They would be a great animal for you, if YOU wanted it. ...but if you are doing it for a 4yr old, it could get old really quick. Especially if you have problems with shell rot or fungal infections or any one of a number of things that might require a
vet visit. Exotic vets tend to be pretty expensive.
If you do decide to go that way, I'm sure everyone here will be glad to help out in anyway we can ...myself included!
I just think that turtles aren't necessarily the best pet for younger kids..
Bob