A four week old kitten shouldn't need any stimulation to urinate and defecate. By that age they should be doing it well enough on their own. In general, cats urinate twice a day and defecate once a day. Of course, it varies as every cat is an individual, but that's the most common scenario.
If they otherwise seem great, then they may not be using the litter box every time and that's why you are not finding anything. You can take the kitten to the box and help it dig around with their front paws and give it lots of praise to encourage it to use the litter. A few brief sessions a few times a day are often all that's needed.
If it is a bit sluggish, their appetite is down, or they are vomiting, then I'd recommend having your vet check them out. A quick feel of their belly will tell the vet if their bladder is full or if they are constipated.
If they otherwise seem great, then they may not be using the litter box every time and that's why you are not finding anything. You can take the kitten to the box and help it dig around with their front paws and give it lots of praise to encourage it to use the litter. A few brief sessions a few times a day are often all that's needed.
If it is a bit sluggish, their appetite is down, or they are vomiting, then I'd recommend having your vet check them out. A quick feel of their belly will tell the vet if their bladder is full or if they are constipated.