If the 4 week old puppy does need bathing you can gently bathe the 4 week old puppy but avoid bathing the puppy too often and only as needed.
You can use a wet washcloth or some puppy wipes to help clean the puppy without really bathing them.
Most puppies unless they get really dirty do not need a bath that often.
The puppies will mostly keep themselves clean but bathing a 4 week old puppy gently is perfectly okay.
Just don't use a whole lot of dog shampoo on the puppy otherwise you could dry out the puppies skin too much.