Is it normal for my 15 month old to only have her 4 front teeth?

Yes: Teeth can be present at birth or not erupt until 15 months. The average eruption pattern starts at 4-5 mo with 1/mo, but later eruption is common & does not mean any deficiency or other issue is preventing it. Often those who erupt later will bring down more than one at a time. By 3 the 20 milk teeth are usually in for all.

4 front teeth: Having only 4 front teeth at 15 months is a bit outside the normal range for teeth eruption. A visit to a pediatric or general dentist would be most beneficial.

At the edge of normal. Standard of Care is that all children have 1st dental exam no later than 1st b'day, whether teeth are present or not. As your child's eruption pattern is slightly altered, best to see a children's dento-facial growth and development specialist, a Pedodontist. Better for your child to have 1st dental appointment without the need to treat.