don't remove them from the mother until they're 6 weeks old, they might end up a bit retarded
i got my two cats at 4 weeks old. the one i still have seemed normal but her sister couldn't do things right... she didn't know how to clean herself (my cat had to clean her), she kept forgetting where her bowl was, getting stuck behind furniture etc.
one day after they'd both been going outside for a while, she never came back. my mum thinks she just got lost or stuck somewhere and died
my cat searched for her for a month but we never did find her