It's not an easy question to answer - it depends on so many things. I drove to Italy several times in the 90s (usually in my F6, but once in a GTL). As you know, one of the many delights of France is its incredible network of D roads - perfect for R4s, but you can't rush things. As soon as you stop anywhere, someone will start chatting to you about the car! I used to find 150 - 200 miles a day doable, though, and you'd see plenty on the way. And France is such a fantastic country for camping!
Some of the high Alpine passes are truly amazing:
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Things are a bit different in Italy - the coast road is incredibly slow, the motorways challenging (especially the tunnels), and avoiding main roads is much harder than in France. There are fewer small campsites too.
If you have the time, it's well worth taking things as slowly as you can - have a great trip!