After second-placed Real Madrid pulled level on points with them on Saturday by beating Leganes, Hansi Flick’s side restored their advantage at the Olympic Stadium.
Lewandowski responded to Madrid striker Kylian Mbappe’s double with one of his own to stay three goals ahead in the race for Spain’s golden boot.
Barcelona, unbeaten this year in 20 matches, took the lead through a Ladislav Krejci own goal near the end of the first half.
Arnaut Danjuma pulled Girona level but Lewandowski netted twice and Ferran Torres finished off the rout.
“We played with patience, we had a plan and if we play with that plan we can always score a lot of goals and win the game,” veteran striker Lewandowski said.
The 36-year-old said he had the legs to keep playing for seasons to come.
“I know a lot of people are talking abut my age but… I want to play more years at the top level,” said the Poland international.
“I feel very good physically, I think looking at the stats there’s no difference now to a few years ago.”
Girona beat Barcelona twice last season but have struggled to hit those heights again and sit 13th.
“They were superior,” admitted Girona coach Michel Sanchez. “There’s a difference between the teams and that’s the truth. Barcelona dream of titles every year.”
Flick rotated some of his usual suspects with the Copa del Rey semi-final clash against Atletico Madrid in mind.
Barca were below their free-flowing best in the first half despite dominating, occasionally threatening through the dynamic running of teenage star Lamine Yamal and frantic energy of Fermin Lopez.
Barcelona wanted a penalty when Fermin was brought down by Yaser Asprilla on the edge of the box and a free-kick was given.
A VAR review showed the contact was inside the area but the referee decided Lopez fouled Asprilla as they came together, much to Barcelona’s chagrin.
Paulo Gazzaniga produced a sensational save with his fingertips to push out a Ronald Araujo header but Barca found a way through before half-time.
Yamal’s dangerous free-kick aimed at Eric Garcia was deflected into his own net by Krejci at the near post.
Girona had produced very little of note in the first half but were back level eight minutes into the second through Danjuma.
Daley Blind played the Dutch striker through and he beat Wojciech Szczesny with a clinical low finish. However, Barcelona were swiftly back in front as La Liga’s top goalscorer Lewandowski notched his 24th of the campaign.
Lopez nodded the ball across the six-yard box and Lewandowski contorted his body to reach it and flick it between Gazzaniga’s legs.
The forward secured Barcelona’s victory with his 25th, slotting home after substitute Frenkie de Jong pushed forward into space and fed him.
There was time for Torres to add a fourth after Gerard Martin found him and for the brilliant Yamal to bend a strike against the crossbar.
This was remarkably the 20th time this season Flick’s side have scored four goals or more from 45 games, with a potential treble in their sights.
“This is what I love about my team, they always want to score goals, it’s always like that,” Flick told reporters.
“This is the mentality and the attitude of the team and it’s good, I think everyone loves it, you love it, the fans love it, the club loves it, and everyone is happy at the moment,” he added.
Elsewhere, fourth-place Athletic Bilbao were held to a 0-0 draw at home by Osasuna, allowing Villarreal to cut the gap on them to six points after beating Getafe 2-1.
Real Betis, sixth, won their sixth game in a row to stay on Villarreal’s tail, beating rivals Sevilla 2-1 in a derby clash.
Valencia’s 1-0 win over Mallorca took them 15th, four points clear of the relegation zone. (AFP)
