Barcelona to Southern France by rental car????

You can rent in Spain and drop in France, but be prepared for a $500-$800 surcharge to do so.

Simplest thing to do is take the train from Barcelona to Perpignan and pick up your rental car there.

There absolutely are a bunch of interesting towns in southern France, not far from the Spanish border. However, there are also interesting towns in Catalunya, on the south side of the border--starting with Girona. They wouldn't require so much travel time.

I warn you that distances in Spain AND France are greater than they appear on maps. For example, Barcelona is in the eastern part of Spain, however it's still 650 miles to Paris.
Have you considered taking a train or airplane up to Madrid? It's a very pleasurable city--far exceeding my expectations on our visit 3 months ago. Inland Spain is completely different than many of the coastal cities which I don't necessarily care for. We did very much enjoy Barcelona, however.

You could visit Madrid or spend more time in Barcelona.
However, another idea, we have spent time in the Costa Brava just north of Barcelona, wonderful towns along the coast, and in pretty, seaside Collioure, France a short distance over the French border with it’s art history and castle.
The Dalí Museum in Fogures is well worth your time, near the Costa Brava.
The Cather Castles are in this part of France too. Also, Carcassonne is north of there. Visit the famous fortified city. Then return car to Spain and fly home.
The second plan would be easy, very interesting
and give you a chance to see places off the radar of the cruise ship itineraries.
Dropping a car off in a different country incurs huge fees. We were quoted a 1000€ drop fee one time. We didn’t do it, rather took a taxi across the border and rented another car.
Save Provence for when you have more than
four days; you need at least a week there, better two weeks.

I have done what Philip suggested, train from Barcelona to Perpignan and pick up a car there. If you want to fly home from Barcelona then get the car there and return it at the airport when you leave. If you spend a few days in southern France you could fly home from Marseille or Nice.

Hello from Wisconsin,
Drive less, enjoy more.

If you are in Barcelona, make sure you really get to see and enjoy it and then the areas around it.

Some one else mentioned how big these nations actually are. I think France and Spain each are equal to five Wisconsins. And the distances are really big if you get off the main roads. But when you are on the four lane roads it is like being here at home. So what is the point of being in Spain or France if you are experiencing USA?

There is a Picasso Museum north/east of Barcelona.

wayne iNWI

Monserrat is a really nice day trip from Barcelona, if you stay in the region. No need for a car- there's trains from the city.

We have spent a great deal of time in SW France and here are some of the places I would recommend!
Quick visit to Perpignan.
Coullioure - beautiful little beachside village where the fauvre artists painted.
Narbonne - very nice town with a spectacular, unfinished cathedral.
Beziers - again, nice town with a cathedral built high on a bluff. The views from the top of the tower are epic!
Pezenas - highlight of the area. Old "village circulade" that has been almost entirely pedestrianized. If you can hit it on market day, it's one of the best markets in SW France. The town gives shop space to artists and artisans on a discounted basis, so it's a great place to shop.
Abbaye del Valmagne - a stunning winery in an old abbey. One of the most unique I've been to.
Montpellier - gorgeous city with beautiful buildings and a culturally rich vibe.
St. Guillaume le Desert - ancient pilgrimage village up a valley along a river. Part of the Camino de Santiago that is not as much used these days. You have to cross a Roman bridge to get into the valley. It's a very unique place and well worth a visit.
These are just a few but will give you some ideas to build a trip around if you like!

If it was me, I'd do what Philip suggests. Train to Perpignan (convenient train service from Barcelona), rent a car there to use in France, drop the car back in Perpignan for a train ride back to Barcelona.

In four days, I'd focus on one area. I love the Languedoc area, maybe my favorite now in France, but Lot Valley is another great choice that's not an unreasonable drive.