My son is 42 inches tall and is 4 years old is that above average for his age ?

Sounds about right: Many kids about 4 years old are about 40 inches tall and weigh around 40 pounds. There is a lot of variation among healthy kids, but the 4 - 40 - 40 pattern is normal for many kids.

Yes: Median height for a 4 year-old boy in the US is just over 40 inches. Coming in at 42 inches puts a 4-year old boy around the 80th percentile (taller than 8 out of any 10 boys his age).