It is a welcome boost for the 26 year-old, whose career has been hampered by injury. He underwent surgery on a ruptured ligament in his right thumb, forcing him out of the title run-in and the Champions League final defeat by Barcelona. He also sat out England's World Cup qualifiers in June.
He remains determined to claim the number one spot when Van der Sar decides to retire but knows he faces stiff competition from Polish shot-stopper Tomasz Kuszczak.
"I'm really happy to commit my future to the club," Foster told the club's official website. "The team spirit here is fantastic and working along side Edwin and Tomasz has been a great influence on my career.'
United manager Sir Alex Ferguson said: "We are delighted that Ben has signed a new four-year deal. Ben is seen as one of the best young goalkeepers in England and we genuinely see him as a successor to Edwin Van der Sar."
Foster, who has been on United's books since 2005, became an overnight hero at Old Trafford earlier this year after he saved Jamie O'Hara's penalty in the Carling Cup final against Tottenham.
Meanwhile, Bordeaux winger Gabriel Obertan is undergoing a medical examination at Old Trafford as he closes in on a move to the Premier League champions.
Obertan, 20, is understood to be joining for £3 million on a four-year contract.
Bordeaux president Jean-Louis Triaud said it was a "great possibility" United would sign Obertan. Bordeaux sent the France under-21 player on loan to Lorient for the second half of last season. He made 15 appearances, scoring one goal.
United has already signed 23-year-old Ecuador winger Antonio Valencia as manager Alex Ferguson looks to fill the goal scoring void left behind by Cristiano Ronaldo and Carlos Tevez.