Driving in Spain doing a Barcelona loop

If visiting the big cities I would not recommend a car, it will soon become a costly liability petrol/parking/IDP. Also Spain is a big country, much more efficient and cost effective to travel by high speed rail for big city travel.

Second Carlos’s thoughts and adding:

If the intention is one of a leisurely, meandering, exploring road trip that includes those cities, 20 days is not nearly enough. Really, that time is sufficient to cover to those 4 plus the train travel between them.

The one section of trip where a car could be helpful is in Andalucía (once you leave Seville), if you want to visit the small towns. Otherwise, like the others, I am not so sure!
On the "plus" side, driving in Spain is very easy, and even the towns and cities aren't too bad to drive to/in compared to other places on Europe.

Compare train travel times on renfe.com to estimated driving times on viamichelin.com. You'll see that the train will usually be much, much faster. In addition, driving-time estimates do not include stops for meals or comfort breaks, traffic delays, navigational errors, etc.

Forget the car, take the high speed train to Madrid.

Do day trips to Toledo and Segovia.
Then take the high speed train from Madrid to Seville. Of course, you can do a stop in Cordoba, but not sure about the high speed train.
https://www.timeout.com/news/this-new-high-speed-train-will-take-you-from-madrid-to-seville-for-less-than-10-022422
High speed train from Seville to Malaga.
https://rail.ninja/route/seville-to-malaga

High-speed train goes through Cordoba. It's difficult to take trains in Andalucia without going through Cordoba--not that one wouldn't want to see Cordoba.

Ditto taking the train if you’re only visiting the large cities. I recently spent 23 days in mostly La Rioja, Teruel, Zaragoza, and Aragón. Since we were visiting small towns and villages not regularly served by public transportation, a rental car made sense. We used Enterprise on that trip and drove 2,900 km total.

Over the last few years, I've spent over 2 months visiting those areas. Twice I've rented a car for visits to the while hill towns (based in Ronda), taking the car only when I left a city and dropping it as soon as I got to the next one. For the rest I used trains and never felt I'd have been better off with a car.