One year we stayed at Tahquamenon Falls SP and then visited Mackinaw, Pictured Rocks, Shipwreck Museum, and Soo Locks. Route 2 travels the southern route of the UP - about 15 minutes (?) from the bridge you get into some great swimming areas where the water is warm.
In the LP I would definitely hit Sleeping Bear Dunes National Lakeshore and do the Pierce Stocking Drive.
Duck Creek is a private park mentioned above that we've hit the last couple summers. They have a nice playground and pool. Across the street is MI Adventure which has both a waterpark and amusement park for one price. The waterpark side is perfect for 4yo and there are a few rides for them in the rollercoaser area.
Do a google image search for Greenfield Village and The Henry Ford Museum and see if anything there would entertain the 4yos .. they're probably too young but they're on the edge.