4 days in Morocco, is Rabat an interesting addition?

If you have four days, focus on Fez and Chefchaouen.

IMHO, no, Rabat is not interesting nor worth your time on a short trip (or even on a long trip, for that matter). Rabat is a dull, ordinary, industrial city. There are much better things to see. I would add that many of us would say that going to Tangier would be a mistake, too - it's the "Tijuana" of Morocco and best avoided.

The "must see" things in Morocco begin with Marrakech and Fes. Flights from Madrid to Marrakech and Fes are dirt cheap, quick and easy. Skip the grotesque, made-for-tourists things in the north coast ferry ports. Fly straight to the good stuff in Marrakech and Fes. With 4 days, fly into Marrakech, stay a day or two, then take the train to Fes. Spend a day or two there, then fly back to Spain. (You could do this in the reverse order, doesn't matter which one you do first).

The low cost airlines do not fly every day, most go every 2 or 3 days. Be sure you check for cheap flights across several days.

Good luck!

I totally agree with David. It's shame to spend any of your limited time in Tangier. You have enough time to get to Fes and/or Marrakech (depending on how the flight schedules work out).

Having spent time in all the Morocco places mentioned here, I do agree that you should consider a change in plans to focus on Marrakech and Fès. There is more of the traditional culture in these two towns than in Tangier. And wonderful restored riads. 4 days is a minimum for these two places, you won’t be disappointed.

Rabat was interesting, but with only four days, I would focus on Marrakesh and Yes. Don't waste a single minute doing anything in Tangier.

If you have time to also go to Asilah, that would be my number one recommendation. It's very close to Tangier and so, so gorgeous. They have an international mural festival every year so it feels like around every corner is yet another beautiful scene. It's one of my favorite places I've been in Morocco, and it makes perfect sense to visit if you'll be in Tangier.

I noticed on another thread that you asked for a riad in Rabat - I'd recommend Dar Maysanne. Rabat has some interesting sites such as Chellah, the Tour Hassan, the Casbah of the Oudayas, and a compact, navigable medina with way better prices than other major cities. You can also cross over to Salé to go to Oulja if you like pottery - funnily enough, it's where just about all of the "authentic" stuff sold in major cities like Fes and Marrakech gets made.

Morocco has many contrasting landscapes, cultures, and environments, and they are all authentic in their own way. There isn't a "real" Morocco. Being a "border town" or a "port city" does not somehow make a place "inauthentic" (and the same goes for Tijuana in Mexico and basically every other country).

Spend your time between Marrakech and Fes, skip border town Tangier.

We really appreciate your input, but for other reason have decided to go to Rabat. Do we need a guide or is it easy enough for tow relatively seasoned travellers to do on their own for two days?

No need for a guide; Rabat is easy to get around in. Even the medina is quite accessible because it's in a grid pattern, so it's much harder to get turned around or lost.