You have really bred a litter without doing your homework???????
What sort of food are you feeding them at present and how often?
If their mother has completely finished with them (not a good thing at four weeks of age) then they should be getting five meals a day - I would feed raw minced meat containing minced bone to start with, then start to add a little pureed veg to the mix a couple of times a day with raw eggs a few times a week. By the time they are five weeks they could probably be well on the way to munching raw chicken wings or carcassas - again with the bone to give them a good source of natural calcium etc etc. Their diet can be expanded to include organ meat and offal, fruit and veg and other raw meaty bones - if you wish to learn more about this diet get hold of Ian Billinghurst's book "Grow Your Pups With Bones"
Christine