My 4 year old constantly grinds/grits his teeth in his sleep. what could be the reason for this?

Normal: Many children grind their teeth at night. It is not uncommon or concerning under ten years old unless your child is literally grinding down his teeth. You can have him checked for pinworms as well..It is not a myth. The worms irritate the anus at night and cause itching that leads to grinding. I do not see this as a common cause for tooth grinding but it is worth exploring.

2 main reasons: One reason is that jaws grow unevenly, a little here, a little there. Kids "grind-in" their occlusions to accommodate. Second reason is they learn so much during the day, they back up the "computer" @ night. Grinding should diminish to disappear as the permanent teeth start to erupt. If occlusion is a problem, remember 1st orthodontic ck-up age 7.

Stress, malocclusion: Bruxism is name for grinding teeth. In 4 yr old can be normal, usually not lasting very long. But could also be due to maloccluded teeth, stress or tension (more so in older child), or acid reflux. This very common condition has not been studied adequately to give better answers. Fortunately it often goes away without treatment. http://www.mayoclinic.com/health/bruxism/ds00337/dsection=causes.