T.,
Please, please get her the helmet and use it. I did not have to deal with this issue with my daughter but I have seen first hand what can happen. I'll try to make this short :)
I have two friends with sons both about to be 4. Neither of my friends had it easy... one boy "John" was born with a club foot and had to have repeated surgeries and wear casts, the other boy "Tom" was born premature and spent the first few months of his life struggling in the hospital.
Both mothers were told that they "should" get a helmet. John's mother choose not to get a helmet and Tom's mother did. John's mother said "he has been through enough and the Dr. said you can't usually tell the difference by age 3-4 anyway". I admit that Tom's mother was upset about him wearing the helmet and didn't like answering questions or the stares that some people gave but she stuck with it.
Well, you absolutely CAN tell the difference. John's head is severely mishapen and almost gives him a "handicapped/special" look. His mother really regrets her decision. I'm sure that she thought it wouldn't be noticeable and therefore not a big deal, but now he has to live with a deformed head for the rest of his life. In contrast, Tom looks perfectly fine. His mom put up with a little aggravation at the beginning and it was well worth it. She used to take it off for pictures and he doesn't even remember that he wore it.
I'm sorry for going on so long... Good luck with whatever you decide to do.
~C.