Short trip with teen

Not much time to cover many bases, so you would really need to pick some interest or main thing you want to see. Four days would be one place/destination. So Art? Food? Beach? Bullfights? History?

Spain because this is a reward for getting an A in freshman Spanish.
4 days because I learned with his older brother that longer than that with a grandmother and no other kids gets boring. (For them, not me)

He enjoys cooking so that would be one thing we could do, and he does like history but too much of that might be boring.

Bullfights! I never thought of that! That would be lots of fun!

Madrid itself would be good, easy to get to, lots to do. For food, there are lots of Tapas/Racione type places where you can try lots of different things. There are also several markets with food stalls where you can get a bit of something, or a bunch.

For things to do, if Art holds no interest, there is a Naval museum. If he is into Soccer, the stadium has a pretty well regarded display, plus you get to go out on the field. Bullfights are seasonal, and somewhat infrequent, but the main arena has an interesting museum and in Plaza Mayor there is a neat bar that has a bunch of bullfight pictures and memorabilia. If you do try to work in a bullfight, be aware, tickets are pricy, and the Spanish bullfights bloody.

Other popular things are the royal palace, you could also day trip to Segovia to see a Roman Aqueduct and a Castle, eat some roast suckling pig.

I suggest Seville, if you can go sometime other than summer.

Our 15-year-old loved the cooking class there. We did a rooftop tour of the cathedral, a tapas tour and a visit to the Alcazar. If he has watched Game of Thrones, scenes from Dorne were filmed there. If you can extend your trip by a day or two, Cordoba was lovely, and it is on the train route back to Madrid where many flights leave from.

If it is summer, avoid southern Spain.