If she had a significant weight loss right before the check up, then that could explain things. 4 ounces is alot when you only weigh around 20 lbs. That would be like losing 2 lbs for a 120 lb person.
If your doctor didn't seem too concerned then it is ok to keep an eye on it and see what happens. If it looks like she is not growing, look further into it.
I seem to remeber my daughter going from 75th percentile for weight and height at 2 months and dropping to 50th percentile for weight and 75th for height. This is where she has stayed pretty consistantly since. Our pediatrician said that it was her genes kicking in. (My husband and I both were also average height string beans. Not so much for my husband anymore! )