His City side beat Brighton 1-0 on Saturday to reach the FA Cup final, BBC Sport reported.
City has already won the Carabao Cup, is in the Champions League quarterfinals and is currently second in the Premier League as it attempts to become the first English club to win all four trophies in one campaign.
"Surviving is a miracle. It's nice to be there," the City manager said.
"Nobody has done it [won the quadruple], so why can we do it? It is almost impossible to achieve everything – that is the truth."
City was below its best against Brighton, but Gabriel Jesus' early goal saw the Citizens reach the FA Cup final for the first time during Guardiola's reign.
The closest any English club has come to winning all four trophies in a season was Manchester United when it won the Treble – the Champions League, Premier League and FA Cup – in 1998-99.
The Red Devils remain the only side to have achieved that feat, but Sir Alex Ferguson's team went out of the League Cup in December 1998 and so missed out on a shot at the quadruple.
"Do you believe we can play 60 games and give a fantastic performance every time? No team can do that," Guardiola said.
"Sir Alex Ferguson won the Treble and not every game was like that. It doesn't work in that way."
City's attention now turns to the Champions League on Tuesday, when it plays Tottenham in their quarterfinal first leg at Spurs' new stadium.
Only two other English teams have still had the chance of winning the quadruple at this stage of the season: Chelsea in 2006-07 and Manchester United in 2008-09.
Managed by Jose Mourinho, Chelsea's hopes of a clean sweep lasted until May 1, but it lost to Liverpool on penalties in the Champions League semifinals. it did win the FA Cup and League Cup, but went on to finish second in the Premier League.
Two years later, Ferguson's Manchester United made it to April 19 before its dream ended with a penalty shootout defeat by Everton in the FA Cup semifinals. It won the Premier League and League Cup, but was the losing finalist in the Champions League.
Last season, City, which broke a series of Premier League points and scoring records on its way to the title, was fighting on four fronts until February 19 before losing to Wigan in the FA Cup.
Celtic is arguably the only team in Europe to have won the ultimate quadruple of domestic trophy, European Cup and both domestic league cups. it picked up the Scottish First Division, European Cup, Scottish Cup and Scottish League Cup in 1967.