We go to Cornwall once a year, usually at half term. Started when daughter was 3.
Places I would recommend
Dairy land - great for a 4 year old. Included a tractor ride, pony ride, animal petting, milking a pretend cow and an indoor softplay. Your 4 year old would not be bored.
Lappa Valley, steam train into attraction, then loads of activities.
Fistral Beach at Newquay -can hire wet suits, have fun in the sea.
Flambards - quite fun, but found it a little expensive for what it offers.
Camel creek - thought this was better than flambards.
Seal sanctuary at Gweck - loved this, if you are into sea life/conservation.
Eden project - we love this, just wandering round, soaking it up. Lots of nooks and crannies for a 4 year old to explore.
Or you could just spend the holiday on the beach - your 4 year old would likely love that just as much.
Enjoy - you will barely be in the caravan!