Hopefully someone will show up soon with better advice, but when I raised some kittens whose mother would not raise them, I stimulated them using a slightly damp cotton ball. Please continue to try very often, because the kitten will not live long if it is not getting rid of its waste.
What are you feeding? Are you feeding cow's milk? If you are, you need to stop and feed Kitten Glop. At four weeks old he should ready to be introduced to the litter pan. It is also possible he is eliminating and you don't notice it- they poop and pee in their beds. How is his stomach? Is it firm and hard, or squishy and soft? Is he the runt? What did the mom die from?
They are eating KMR weaning formula, Royal Carin Baby Cat, and Nutro canned food. They have been introduced to the litter box, only one uses it regularly. His belly is hard and he looks like he is straining. He is not the runt. There are 3 kittens that weigh less than him. The vet believes it was a blood clot that killed there monther.
Constipated or straining kittens? In this case, you would want to increase the strength of the formula, and feed slightly smaller amounts, but feed more frequently. If the kitten has a swollen abdomen and hasn't passed a bowel movement in over a day, you may want to try mineral oil given by mouth (3 drops per ounce of body weight). You may want to discuss with your vet whether or not a warm, soap-water enema can or should be given.