4 yr olds stool is black on outside almost like a shell, but normal brown on inside. he is mostly constipated.

Fix the constipation: Each person with constipation needs to be sure there is no other disease causing it. If constipation is just due to slow movement of the food and poop through the gut, then the things to try include: vegetables, prune juice, prunes, Miralax, (polyethylene glycol) Senokot or other senna, milk of magnesia, other pediatric laxatives... His doctor should follow along, to be sure that the stool's appearance normalizes.