suggestions for four days in Southern France from Aix-en-Provence at the end of June?

Many of the cute towns in Provence are not reachable by train, and buses were infrequent when I last attempted them. The ideal train route is that Avignon toward Lyon to the north, and toward Arles, Nimes, ..... Narbonne to the south. It seems I took the train from Avignon to Aix 12 years ago and it only took an hour. But more recently, it took 2 hours each way - either divert to Marseille, or take the TGV but the getting to and from the TGV stations was time consuming.

You can go down to Toulon, Antibes, Cannes, Nice, and Menton. Those are all very train friendly and close together.

I'd give Aix another day after the tour ends, then train to Marseilles for the rest of your time. You can fly from there to Paris or wherever else works for returning home. Or take the same train to Marseilles and then a TGV to Nice, with flights to Paris and elsewhere.

I just came back from a month in Provence and after spending a week in Aix and Arles, spent 2 in Avignon as there were 12 towns for me to visit and planned them so I did them om their market day! F, S, 70+ solo traveler and only traveled by train and bus. Bus very cheap, mainly 2.50E one way.

Pont du Gard, L'Isle sur la sorgue, Roussillon, Orange, Menerbes. and Villeneuve. First 4 are musts in my opinion. Menerbes was Peter Mayle's heaven in France and if you haven't read A Year in Provence you will be entertained, amused and laugh out loud and because of that had to go there. A bit of a logistical problem as I went in November. Villeveuve is across the Rhone and ties up the history of the area with Avignon.

I used rome2rio.com to get a sense of how to get there and then finessed it with the actual transpo companies. Other towns to see but depends on what you did in your previous visits. Can say a lot more but will leave it at that!

End of June is actually prime time for the lavender fields. You can probably find a tour from Aix to the Valensole area if you do not want to rent a car for the day.

I have found multiple day trip opportunities from Aix. The lavender tours seem to focus on Plateau de Valensole and the Luberon village tours mostly go to Lourmarin, Bonnieux, Roussillon, and Gordes, before returning to Aix. I was looking at Viator, but the other tour aggregator sites have similar offerings.