okay so if you know what you are looking for ....

... can you really tell a rabbits sex at 4 wks?

I have just had a lady to come and collect Smudge and she has paid for one of the babies as well as Smudge. She already has a neutured boy and smudge was going to be a friend of him however Smudge is a boy even though I was told she was a girl! This lady then looked at the babies (4 wks) and told me that 3 are girls and 1 is a boy! Is it possible to tell at this age? good news if she is right as Angel is the boy and Holly is a girl, which means they should live happily together

Yes, it is possible to tell at that age, but even those who know what they are doing can get it wrong, so I personally wouldn't focus completely on what she said as being right, maybe just bear it in mind and keep checking yourself. Breeders seem to say that they can tell at birth because a girl dribbles her wee and a boy will squirt it, but that's not personally something I have experienced.