Four days is the sweet spot for a first visit to Barcelona, turning a sightseeing sprint into an actual holiday. Days one to three cover the essentials: the Gaudi trail (Sagrada Familia, the Passeig de Gracia houses, Park Guell), the old town (the cathedral, El Born, the Picasso Museum, La Boqueria, La Rambla), and a mixed day of the concert hall, Ciutadella park, and the beach, each done without rushing. Day four is the release valve, built around Montjuic (the MNAC, Miro, gardens, and castle, reached by funicular and cable car) plus the Magic Fountain, or swapped for a day trip out to Montserrat's mountain monastery or Girona by fast train. This length suits travelers who hate the forced march, couples, and anyone whose flights bookend the trip awkwardly. For four days we recommend basing in Gracia: you have accepted a slower pace, and its plazas and family-run bars make coming home each night the best part of the day, with Park Guell up the hill above you. Book Sagrada Familia, Park Guell, and the Gaudi houses well ahead, and keep your bags zipped in the crowds.