Just read your previous comment...potty training can cause some anxiety for some children
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...that would not solve the problem, but may minimize any damage that may be done to his teeth.