We had a good stay with kids in Hannibal, lots of stuff to do in walking distance on the riverfront there.  LeClaire is awesome in the Quad City area too.  4 year old may be a little more limiting on what places you can enjoy than a 4th grader would be, though.   Some good places you could do in Burlington, too.  Email me or Bookface message if you want any more specifics on some of those towns.