Barcelona took the lead in the ninth minute when Ferran Torres scored past Espanyol's goalkeeper, and the same player doubled the Catalans' advantage in the 25th minute, setting the halftime score.
In the 54th minute, Torres had a goal disallowed, missing the chance for a hat trick, before Pol Lozano reduced the Catalans' lead just two minutes later.
By the end of the match, Barça scored two more goals to secure victory in the city derby. The scorers were Lamine Yamal and Marcus Rashford, extending the Catalans' lead over Real Madrid to nine points.
The Catalans now have 79 points and lead the table, while Espanyol sits tenth with 38 points.
In the next round, Barcelona will host Celta Vigo, while they will also travel to Madrid to face Atlético, aiming to overturn a 2-0 deficit from the first leg played at Camp Nou. Meanwhile, Espanyol will visit Rayo Vallecano.
Score and scorers: Barcelona - Espanyol 4:1 (Torres 9', 25', Yamal 87', Rashford 89' / Lozano 56')