Both Lisbon and Seville are wonderful cities and would be great short-breaks. Both have much sightseeing opportunities to fill four days (and easy day-trip options), good food, and a relaxing ambience.
You can check weather averages on-line but I suspect Sevilla is usually more Winter sunny, a bit hotter in the daytime and has fewer rain days than Lisboa in February. However, there's no way to predict with certainty and to some extent it's the luck of the draw. We were in Lisbon for few days late in February a couple of years ago and the weather (for Winter) was brilliant. We were in Seville last year in February/March and it was even better. But who knows next February?
From Madrid, Sevilla is faster to get to than Lisbon since you can take the high-speed "AVE" railway service. However, it partly depends where you will be flying into and out of. Lisbon has direct flights, I think, to the Americas. Seville doesn't. If you choose Lisboa, then I'd recommend taking the sleeper service from Madrid rather than submitting yourself to the faff of short-haul flying between the two cities. Train is a much more pleasant way to travel.
For me, Sevilla is Spain's best city and one of Europe's top five. So, if forced to choose, my choice would be 3 days in Madrid, followed by 5 days in Seville (or 4 if you need to get back to Madrid to fly home). If you do choose Seville, then an option is to stop-off for a day in Cordoba when travelling from or to Madrid. Cordoba is a fabulous destination (actually well-worth an overnight or two if you had longer).
