teeth grinding is something that can be fairly common for
children and adults. Often, people grind their
teeth when they are stressed out about something or anxious. It can be difficult to address this issue with a 4 year old though. Are you aware of anything that may be causing some anxiety? A new routine like beginning pre-school, a move to a new location, house, etc?
If it really becomes a problem, you can take him to the dentist and they can fit him for a mouth guard...