Labial adhesions are felt to be caused by mild irritation of the labia, either from diaper rashes, infections, poor hygiene (three- and four-year-old girls are notoriously poor at wiping), or just excessive rubbing. As the irritation heals, the edges of the labia stick together and become fused. If you look closely at it, you can usually see a thin clear line down the middle, which is the thin membrane that has formed.
It's estimated that up to 10 to 20 percent of girls have these labial adhesions at some time in their first six years of life. For most girls, these adhesions resolve on their own in six to twelve months, without any treatment. For girls in whom the adhesion covers a very large area, or who have some difficulty with getting their urine out, estrogen cream is the recommended treatment. Estrogen works because it protects the labia from irritation (labial adhesions don't occur in girls once they have started puberty). The estrogen cream is usually used twice a day for two weeks, then once daily for another week or two. The cream is very safe to use for these short periods of time. It is usually quite successful in separating the adhesion, although occasionally, some girls need to be retreated. Recurrence of the adhesion can occur, so it is recommended that some sort of lubricating ointment like petroleum jelly (Vaseline) be put on the recently separated labia, to prevent them from sticking together again.
You should talk with your pediatrician about whether or not your daughter needs to be treated further, and for how long.