Jonathan Spector and Carlton Cole scored two goals each at Upton Park as holders United were beaten in the competiton for the first time in more than three years.
Pounce: American Jonathan Spector grabbed a shock brace against United
Sir Alex Ferguson was stunned by the result, hitting out at the "terrible goals" conceded by his team and at the physical approach of the Hammers.
Ferguson said: 'I didn't expect that, that's for sure. If you analyseit the goals we gave away were absolutely too soft. They were terrible goals.
'They were young players, too young to carry these types of mistakes. They're 20, 21 years old. They can't make mistakes like this and still win.
'The young players will remember it but this could have been a good opportunity to win the cup again. We were in control in the early part, played some good football. But goals change games.
'They were well fired up, they made a lot of challenges, that's the way they approached the game and it's understandable.'
Stunned: Sir Alex Ferguson was disappointed with the manner of his side's defeat
Ryan Giggs claimed it might have been different had Robert Green not tipped a shot from Gabriel Obertan on to a post in the opening minutes.
Giggssaid: 'We were disappointed to lose 4-0. We were confident we could win. West Ham played well and scored some good goals but we feel we could have defended a lot better.
'We started well and that shot from Gabby might have gone in, but after that they scored quickly and it probably knocked the stuffing out of us.
'Our heads maybe dropped but we are Manchester United and we have got to rise above that. We have got to carry on playing.'
West Ham boss Avram Grant hopes the result can help inspire a move out of the Barclays Premier League relegation zone.
Happy Hammers: Avram Grant congratulates goalscorer Carlton Cole
'I thought we could win but this is still an unbelievable result. We continue our progress,' he said.
'We scored four goals and could have scored more. But for me four is enough. To be in the semi-finals is very good for this club. We are trying to build something step by step. We have done the right thing.
'We have played many more good games without winning so the players maybe have lost confidence.
'But the most important is to show mental strength and tonight was so unbelievable. The gap at the bottom of the Premier League was five points and is now three points.
'We have many games left and I'm sure that this shows we can be good in the future.'
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