South of France- Marseille/Arles/Cassis/Avignon 4 days

With only 4 days and your objective being able to explore the south of France, you don't have enough time.
We can help you more if you tell us more about this segment of your trip: for example, just train or are you going to rent a car?

Reading between the lines I am assuming you wish to do this journey by train with 4 nights (5 days??)
Day 1 Train to Marseille (2nts)
Day 2 Excursion to Aix-en-Provence
Day 3 am Train to Arles (2nts)
Day 3 pm Excursion to Nimes (& Pont du Gard)
Day 4 Excursion to Avignon (& Orange)
Day 5 Return to Barcelona
This is the local train map to help you ... http://www.beyond.fr/map/ter_paca.html

Sorry, I didn't mention that travel would be by train and I know that all of these is tight, so which would be most advisable to visit? Thanks!

I think you could fit them all in as I suggest - they are all close and travelling mid summer you will have lots of daylight. You will only need a light shoulder bag - dump it at your Arles hotel on Day 3 and head on to Nimes for the afternoon..

The only problem with the above is that you are missing Cassis, which is a spectacular site, not just the Calanques but Cap Canaille, too. Instead of stopping in Marseille, I suggest you change in Marseille to the local train to Cassis for the first night. The next day you can catch a bus between Cassis and Aix. I didn't check the hours and if it runs daily, but my son has taken it. Then continue on as set out above. You'll miss Marseille, but you'll be getting plenty of other cities and towns in exchange for the spectacular scenery in Cassis.

I stayed in Nimes for several nights,misery easy transportation links to other cities. The Pont-du-Gard is a wonferful place to visit. Bets' advice for Cassis is great.

Yes the beach might be nice for a day if you are in the Marseille area, I just automatically think countryside for Provence. Pont du Gard (probably by bus from Nimes) is definitely a must.