Goals from Craig Bryson. Chris Martin and Richard Keogh put Derby in control before Grant McCann converted a penalty after goalkeeper Frank Fielding was red-carded in the 61st minute.
But Charlton's late winner against Wolves kept Peterborough out of the relegation zone on goal difference and 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. They were outstanding and thoroughly deserved the win.
"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 here we are down, if we play like we did for the last 10 games before this 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."
- Sports & Recreation
- Frank Fielding