A four week old kitten is too young to pee in a litter box. At that age, kittens still require urogenital stimulation to produce stool and urine (they will eventually pee when the pressure reaches high levels but it is not healthy for them to wait that long). Stool production is especially important – they can get life threatening impactions if they are not stimulated regularly. If it is actually older than four weeks (or particularly advanced), it may be peeing and pooping on its own, but it doesn’t have much control over where and when. You can keep a clean litter box with very low sides in the area of the kitten and it will learn to use the litter on its own…....when it’s ready. No teaching involved.
If you are not familiar with hand raising neonates then I suggest that you take the kitten by your regular veterinary clinic for lessons immediately.