I did it when I was in my late teens and loved it and agree that there is a lot of hanging around etc, but I don't see why you couldn't tailor it to suit a 4 year old. (I have one too so can easily imagine...)
Boat to Le Havre, loads to do on the ferry, no problem. Couple of hours train to Paris, OK. One day in Paris, I'm sure s/he could manage with the promise of Eurodisney the next day! 4 or 5 hour train to Lyon?/Geneva?/Bordeaux, one day in a city, one day at the beach/lake etc etc. If your friends were willing of course.... which is probably the hardest part.
The longest train journeys we did were about 8 hours apiece, so I would definitely break that in two. 4 hours on a train is easier than 2 hours wait to board a plane, then an hour flight, then passport control etc.