Four weeks old is kind of young to be worming kittens...but without the mother available, I can understand trying.
I have noticed in kittens with feline leukimia that kidney failure at about 3-4 weeks is common. Which is not to say this is the problem. It could be a result of not getting mother's milk (necessary vitamins, natural antibiotics, etc...) which can't be helped in a situation like this.
Unfortunately, working with a shelter generally meant kittens that showed signs of illness under 10 weeks were put down, so I'm not sure what to suggest (one of the reasons we ended up sticking with older cats, stray kittens have a very high expiration rate and frankly, working with the kittens started to be depressing. Plus, the healthy ones didn't need us, they adopted out quickly).
Any one else got any other ideas on what may help out there?
try some of the info there.