Posh beat the Championship promotion-chasers 2-0 thanks to goals from Joe Ward and teenage substitute Ricky-Jade Jones.
But there will still aspects of the performance that frustrated Ferguson, although he hopes the confidence from beating a talented in-form team will be a platform for a surge away from relegation danger in the Championship.
“It was a great result for us,” Ferguson said. “It was something we needed after a difficult month. We kept a clean sheet which was also needed and we beat a very good side who have been in very good form.
“In the second-half we played like I want us to play, with intensity, pressing and aggression and we scored a high quality goal from Ricky.
“Our substitutes made a huge difference. Reece Brown gave us control of the ball, while Ricky, Sammie Szmodics and Hayden Coulson gave us the energy we lacked in the first-half. Jeando Fuchs and Oliver Norburn were very good off the ball.
“It was a strong, resilient, determined performance all-round and the result was the most important thing as it always is.
“I am happy, but I’m also frustrated. We rode our luck a bit in the first-half from set-pieces and we had nowhere near enough energy. They were far too comfortable in the first-half and that started from us up top. There was no pressure on the ball. We have to get the ball forward into our strikers, but it didn’t happen, It’s probably a confidence thing. I made a change to sort that out at half-time (Szmodics replaced Jonson Clarke-Harris) and we were much more like ourselves.
“And we could have conceded within 30 seconds of the re-start which is a nonsense. We have to start halves better or we will be punished,
“But we showed there is plenty of fight left in us and I’m pleased for the players as they‘ve had a tough time. It must be a platform for us now as we approach a crucial month.
“I’ve told the players they have to start showing what they are about. We have to show that we are much better than we’ve shown so far this season.
“What I want now is a good away draw for the fans. They’ve endured a terrible season so far so a big day out at a big club would be ideal.
“Knowing my luck we will get Stoke or Huddersfield away though!”
The draw for the fifth round is scheduled for about 11.40am tomorrow (February 6) on ITV before the Liverpool tie against Cardiff City at Anfield.
Posh continue their relegation battle at Cardiff on Wednesday (February 9).