It is hard to say. The one who found the cat could lay claim to the mother, and thus claim all the kittens as well. You could all claim ownership of the cat since it has been a joint effort in taking care of it. All four of you obviously could not split the original cat, so your best solution is to divide up the kittens and then each of you can do what you want with them. This is probably not a question that the law can help you answer.