We are going to Madrid for 4 days and then would like to go somewhere else for four days-We are thinking Sevilla. Any suggestions?
Sevilla works, but research other cities too. Select a city because there’s something there that you want to see, not just picking it at random. Other popular cities include Bilbao, Barcelona, San Sebastián, Santander, Granada, Cádiz, Burgos, León, Cáceres, Valencia, and others. Only you can decide which would be worth a four day stay.
Jaime's answer is very good, and I would only add, "when is your trip?". I would try to avoid Sevilla in the summer.
We spent 11 nights in Madrid and Andalucia -- including Sevilla. Our trip report might be useful reference: https://community.ricksteves.com/travel-forum/trip-reports/trip-report-spain-aug-sep-seville-ronda-granada-cordoba-madrid