The Premier League leaders eased to victory over the Swiss champions at St-Jakob Park with midfielder Ilkay Gündogan netting a brace, while Bernardo Silva and in-form striker Sergio Aguero each added a goal.
With four away goals in the first-leg, the Citizens would need to concede five or more goals without scoring against RotBlau in the reverse fixture at the Etihad, to be eliminated from Europe's elite club competition.
"It's an amazing result in the last-16 of this competition," Guardiola said, as quoted via The Express. "The Champions League is 180 minutes, but we are almost there.
"I'm not saying it's done because in football anything can happen, but it's an amazing result. I'm delighted, and now we prepare for Wigan."
City face off against the Latics on Monday in their fifth round FA Cup tie, as their quest for the quadruple continues.