Four days is more than enough for Lisbon, and the fourth day is what turns a sightseeing sprint into an actual holiday. Days one and two cover the historic core: castle, Alfama, tram 28, Baixa, Chiado, the miradouros, and both signature nights out. Day three is Belem, done properly from the monastery's first entry slot to dinner at LX Factory. Day four is the release valve: the Gulbenkian's world-class private collection, the gardens and wine bars of Principe Real, and an unhurried final evening, or swap the whole day for a Sintra day trip if palaces tempt you more than museums. This length suits travelers who hate rushing, repeat visitors, and anyone whose flights bookend the trip awkwardly. For four days we recommend basing in Principe Real rather than downtown: you have already accepted a slower pace, and its restaurants, garden squares, and local crowd make coming home each night the best part. Beyond four days, add Sintra and a Cascais beach afternoon rather than more city.