Pretend for a moment. that you are a TENANT, and your landlord wants to come into you house and check it out 4 times a year: After the second or thid time, you'd be screaming at him.
NO! Don't do that!
You have a rental agreement that spells out details like damage to the property, responsibility for repairs, etc.
You cannot oversee how clean or neat they are. You cannot criticize if they have dirty dishes in the sink or the diaper pail has not been emptied.
You will drive yourself crazy with this level of surveilance. You don't want to know.
When they move out, you will clean it and repair any damage; you accepted this when you decided to become a landlord.
I would not be surprised if there were ordinances limiting how how often you could legally encroach on their privacy.