I have a four yr old and when she gets hot her nose starts to bleed a lot what's the cause?

Kids get nosebleeds. Nosebleeds can be due to many things that cause one of the little veins on the inside of the nose (usually the middle septum) to crack and leak temporarily. Heavy breathing in sports, hot/cold air, allergies, nose-picking, colds, a toy in the nose, etc... Are some of the causes of nosebleeds. She who bleeds "too often" or "too long" can have her blood checked to see if her blood clots too slowly.
Eval. Are you in a dry climate? Does she pick her nose often? Has she had a recent upper respiratory infection? Maybe a small blood vessel in the nose is the cause? Regardless, frequent and recurrent epistaxis (nose bleeds) warrants medical evaluation. Please have her seen by your pediatrician.