My 4 year old daughter's top front tooth has fallen out. Is this normal?

Baby tooth. The typical age for the upper front teeth to come out is about 7 so your daughter is not typical. Baby teeth can come out early due to trauma, decay, or she could be growing atypically. Take her to a dentist and have her evaluated and make sure there are. It problems that can effect her permanent teeth.
Not really. The "normal" age range is from 6 to 7 years old, but variation can be wide. 4 years old is kind of young though. I would suggest a visit to a children's dentist (they're called pedodontists) for a consultation.
Tooth Loss. If she was 7 yes, if she is 1 no and you should see a dentist to determine the cause. Is the root intact? Was the area traumatized? Is there decay? Baby teeth guide the eruption of the permanent teeth. See the dentist.