My 4 yo often has blood following a bowel movement. What could be causing this in a child?

Couple of causes. In children that age, a rectal fissure is a common cause of bleeding following a bowel movement. Often they have some stool withholding, leading to large stools. This can lead to a fissure when passed. A polyp can cause a large amount of painless bleeding in children. If it has been going on for a while, i would suggest seeing your doctor.