My 4 year old just got over the stomach flu and now has a rash all over his entire body that looks like goosebumps and is itchy. What could this be?

Most likely... After a viral infection which seems this was the case a rash could appear. In some cases it could be itchy and you can use Benadryl (diphenhydramine) OTC to relieve symptoms of itching. If he looks sick take him to see his doctor.