How much to feed 4 to 6 week old kittens?

As much as they are able to eat. At 4 to 6 weeks old, kittens are generally still suckling from their mother, or, if their mother isn't around, being fed from a bottle. Young kittens are usually fed every couple of hours.

It is not unknown for such young kittens to start to take an interest in their mother's food, although 6 to 8 weeks is the more usual time for weaning. Have a shallow dish of water out, and offer small portions of mashed-up wet cat food as frequently as you are able. It is best to put down food for them (on a plate, not in a bowl is a plate is easier to eat out of) at least six times a day.