Piggies that are going to be rehomed can leave at approx 6 weeks - although they can be separated from their mother at 4 weeks they are too young to have the stress of going to a new home - another few weeks will let them grow that little bit more and become more robust.
If you are rehoming the babies it is a good idea to write up a care sheet for the new owner and provide a list of good guinea pig books and websites where they can find information. Also provide them with samples of food & bedding - it is a good idea to offer to look after the babies when the new owner goes on holiday (so you can check on them!!) and to offer a guaranteed home for the piggies if the new owner can no longer provide for them. Try and stay in touch if you can to put your mind at rest.