My 4 yo son (closer to 5 ) is about 50 lbs these days. I wouldn't worry too much that your child is "off the charts" - every child is different.
My son's head was so huge when he was born, we had to get a cat-scan. His head circumference is larger than mine now, but he's "normal" in every other way except that he wears adult hats - no biggie.