Juan Mata described Manchester United's 4-2 derby victory as a 'massive win' as they opened up a four-point gap on Manchester City. United had lost their last four meetings with City but goals from Ashley Young, Marouane Fellaini, Mata and Chris Smalling gave the Red Devils their biggest derby win for six years. Sergio Aguero opened the scoring and also netted late on for his 100th City goal but United were by far the better team to tighten their grip on third spot in the Barclays Premier League. Old Trafford roars with Marouane Fellaini after the influential Belgian gave the Red Devils the lead . Wayne Rooney leaps onto Fellaini's back after United take the lead at Old Trafford in the Manchester derby . 'The first 10 or 15 minutes they were brilliant and played well but after that the team showed character and we came back,' Mata told Sky Sports 1. 'It's a massive win for us, it's a great result and everyone at United is happy. 'The atmosphere was crazy, it is something we will never forget. Juan Mata, released by Rooney, scores as Manchester United smash their rivals at Old Trafford . 'You could tell how important it was for the fans but we still have six games to go and we have to keep going. 'It's a big win and a boost of confidence but Arsenal are doing great and I'm sure City and Liverpool will fight to the end, so we have to fight to the last minute.' Young equalised Aguero's early strike and then set up Fellaini to give United a first-half lead. All of United's outfield players congratulate Chris Smalling after his close-range header ensured victory . 'We were disappointed to go a goal behind but we showed that team spirit to get back in the game and no other team was going to win it when we went ahead,' Young told Sky Sports 1. 'It was an amazing feeling to score. I was delighted to put the ball in the net and get an assist as well but the important thing was we showed that character to win the game. 'It was a magnificent win. We knew it was going to be a tough game but we knew we had to get the three points.' Ashley Young latches on to a loose ball which bounced off Gael Clichy and fires past the despairing Joe Hart .
Juan Mata scored as Manchester United thumped Manchester City .
Victory sends Louis van Gaal's side four points clear of their rivals in third .
Manchester United are now just one point adrift of second placed Arsenal .
Mata has challenged his teammates to keep on fighting right to the end .