Is 90 pounds too much for a fourth grader?

That depends on how tall they are and how fast they are growing. Some girls start puberty in 4th grade and start to gain more weight so that could have someting to do with it as well. It seems perfectly normal to me. I was about 90 pounds in forth grade and now I am in 9th grade and weigh 112 lbs so I wouldn't worry about it.