I agree with Cataholic...I don't think kitties should be taken away from their mom until 6-8 weeks.
Not being an expert and not knowing everthing a mommy cat needs to teach them, but I am guessing those two weeks can make a difference with socialization and perhaps overall health with respect to nursing and the nutrients transferred from mommy to baby.

My Slushy was given to a shelter at 4 weeks and I just had to adopt her because she was just too small to be on her own in that nasty ol shelter (my motherly instinct took over). Although she was easily litter trained...my older female cat had to show her "the ropes" on little things...