Derby netted three times in the opening 52 minutes - through Craig Bryson, Chris Martin and Richard Keogh - before Grant McCann converted a 64th minute penalty after home keeper Frank Fielding was shown a red card.
Posh are above the relegation zone on goal difference only but Ferguson said: "I'm absolutely delighted. We are out of the bottom three which is the best thing that's happened all day.
"Nothing else positive did happen so it's still in our own hands with two games to go and after the start we had to the season, losing our first seven games, I would have taken that.
"Sometimes as a manager you have to hold your hands up and praise the opposition and I thought Derby were magnificent.
"We just didn't get out of the traps, we didn't get going really until it went to 3-1.
"If we play like we did today we are down, if we play like we did for the last 10 games before today we will stay up.
"There are no grey areas, it's black and white. I have to give the players praise for the last 10 games but we were beaten by a better team today."