Dairrhea can be caused by a bunch of things and if it lasts for more than a day in really young animals, I call the vet. It could be parasites, intestinal bug, reaction to food or if they had their boosters recently. I too, may be handraising a 4 week old kitten (Hurricane Isabel victim and if she is FeLeuk/Fiv neg, she can stay – I have five other cats to worry aboutand my vet can see her tomorrow afternoon). Just about set to wean but still needs supplementing with KMR a few times a day at this age. Babies can go down fast and if the diarrhea has been for a day or more, mucousy, bloody or very frequent, get them in for a check. Dehydration is the big concern even if there is no actual cause – but you have to see if there is and bring in a sample of the diarrhea as well for the vet to check.