Cesc Fabregas netted twice in the opening 18 minutes at the Nou Camp - the second a superb chip, before Lionel Messi added two more goals, climbing off the bench to net in the 72nd and 90th minutes.
Speaking after the match, Guardiola praised his side's display but warned against complacency - citing last season's 3-1 defeat to Real Betis in the quarter-finals as an example.
"In general terms we played an excellent game and we've made a positive step forward in the playoff," the Spanish coach told the club's official website.
"But it's not decided yet, because we his bout of flu must go play in Pamplona. We still have the memory of the 5-0 over Betis and then suffering in the return leg fresh in our minds. The tie will be over after we've played 90 minutes."
Intially ruled out of the match with a bout of flu, Messi managed to recover and brought his tally to 31 goals this season with a double.
Guardiola added: "This morning [Wednesday] he has a stomach ache, he was in pain, cold and had a small fever, that's why we decided that he should stay home. Afterward, he called and said he was feeling better.
"This morning [Wednesday] we wanted to announce that he didn't train and that he'd been ruled out. However, I left him on the bench, we hadn't closed out the game and I decided to put him in for half an hour. We know that we're a little bit better with Messi."
Barcelona resume their La Liga campaign away to local rivals Espanyol on Sunday.