(Fr Riviera?????) after 4 days with rental car in Provence.....HELP!

You can take a 3.5h train from Avignon to Nice and spend a full day exploreing Nice. Don't miss Chagall's Museum and the Promenade walk. I would also buy RSEs Provence & the FR Riviera guide which you can also download from his app.
On day two take a 30m train to Monte Carlo and spend the day exploring Monaco. Day three take a 10m train to Villefranche-sur-Mer for the day. What a beautiful vacation to look forward to.

I spent a couple of weeks in Nice (taking language classes) and enjoyed a lot of side-trips along the Riviera. I didn't see disgusting displays of wealth, and I didn't find the area particularly expensive. But I didn't go to Monte Carlo, Cannes, or St-Tropez, which may be more ostentatious. The folks in Villefranche-sur-Mer told me that a lot of the cruisers head straight to Monte Carlo. My trip was in May 2017, and I doubt that conditions will be more crowded for you in October 2020. That's pretty clearly not high season for the Riviera. So there should be lots of places that aren't mobbed. However, the southern coast of France is very popular, so you have to go pretty far afield to be the lone American or English-speaker around; you can do it if you want to focus on that characteristic rather than going for the best-known beauty spots or most popular museums.

I second the suggestion that you pick up Rick's more detailed guidebook that focuses on southern France. Read about the various towns and see what appeals to you. There are a couple of gardens in Menton that I really liked, and Menton has a very nice medieval district--unfortunately extremely touristy. There are lots of great art museums along the Riviera. There are hill towns, some of which are touristy (St-Paul-de-Vence and I guess Eze, which I haven't seen) and some are not (Vence, but it doesn't have the fabulous location the other two have).

But you say you have "3 days". Where must you be at the end of the third day? What happens on the fourth day? I'm thinking that you'll spend over half a day relocating from Provence to the Riviera. Your car will be of no use to you along the coast (good rail service, bad/costly parking) but would be helpful to visit the hill towns. What to do?

You could easily fill a full week in Provence, and you wouldn't lose a major chunk of one day getting over to Nice or other selected Riviera spot. If you're heading back to Paris, the train trip from Provence is likely to be a lot shorter than the trip from Nice.

The reason why all those rich people pack the Cote d'Azur is that it is a spectacular location. The scenery is the star of the show. Best road trip: the coast road, with stops at St Jean-Cap Ferrat & Villefrance-sur-Mer. The Moyen Terrace Road-- half way up the cliff also has great views-- but the not to be missed sights are the Perched Villages (Eze Village, not Eze sur Mer) and its neighbors. For your itinerary, the splurge meal is lunch at Chevre d'Or in Eze reserve a window table).

The Riviera has many appeals, but so does Cassis. Cassis has one advantage : it is much closer! And, while there are more spectacular villages in the Riviera, the coastal scenery is the most dramatic of the French Mediterranean coast.
It's not quite a 3-day place though ; 3 nights/2 days is plenty already, so you could add a day to the rest of Provence, which deserves it.
If you decide on Cassis, keep the car.

Four days is very little time for Provence so you could spend it all thete.
Or go to Nice to use it as a base, drop off your car at airport when you arrive. There is excellent transportation from Nice for day trips our favorites were Cap Ferrat and Antibes. Don’t waste your time visiting Monaco,Monte Carlo.
Nice is a wonderful, small city.

You've gotten good suggestions. Just adding that the coast doesn't have all that much bling, except a couple of places, but it does have beautiful scenery and way of life. We can't believe everything we read.

The only areas where wealth becomes ostentatious is in a section of Monaco/Monte Carlo, a section where many cruise passengers go. There are yachts in the harbor as well. There are also certain shopping streets in Cannes where luxe is on display. Otherwise, things are pretty normal, with the only evidence of a stronger tax base being clean and well maintained streets and buildings, and investments in infrastructure.

In October, there are fewer tourists than in May. The most touristy areas tend to me the most popular villages, but really no different than certain villages in western Provence (like les Baux) with the exception of the cruise ships that make stops and cause spikes (primarily in Monaco and Villefrance-sur-Mer).

Cassis has the Calanques, which are supposed to be incredible! And this exhibit in Les Baux will be open, also supposed to be incredible! https://www.carrieres-lumieres.com/en/home We will be in Aix-en-Provence in September/October and both are on my list :)