Nursing mama cats tend to be pretty accepting of babies–hence nursing cats who take on puppies and skunks and the like. Milk supply tends to follow demand so as long as mama cat is getting plenty of food and water to keep her topped up her milk supply should be more than adequate. A litter of four isn’t excessive so one more shouldn’t strain anything even if the new kitten is a little older. Best way to see if she’ll accept the newcomer is to try it–bring the new one over and put it near the nest box and see if mama comes to grab it and take it in herself, which is pretty likely, especially if it’s mewing. Just make sure new kitten doesn’t have any fleas on it before putting it in with your litter. Worst that happens is that mama thinks it over and says “Nah” then it’s just back to status quo.
Yup; about the only way to tell whether a particular mama cat will accept an additional kitten is to ask the mama cat. Definitely worth trying, though – especially if she’s acting anxious about hearing it mewing.
Five kittens in a litter shouldn’t be anywhere near too many for a well-fed cat to nurse. And if mama’s willing, the kitten and the neighbor will both be better off. It’s difficult (though not impossible) for a human to do the work needed just to physically keep a kitten that young alive; and mama cat (plus the rest of her litter) will do a much better job of teaching the kitten how to be a cat.
If it’s any help, the Tiny Kittens rescue out in British Columbia has had major success in having nursing mothers accept other kittens. At one point, one mamma was feeding two litters, totalling, I think, 10 kittens, because the other mother’s milk just wasn’t coming in.
Just copypasta the link to the applicable pics here in a thread and we can follow them ourselves, no invitation needed in imgur. No account needed either, you can just drop an image into the upload window and get the URL immediately. Imgur is nice like that.
I love that channel! They had a webcam a while back that featured 3 litters born within a week of each other, and two of the mothers co-nursed all the surviving kittens. The third mother was taken elsewhere because she was ill and lost all her kittens except one, and they all felt they were better off with her being quarantined.
The only way to find out if this mother will accept the kitten is to bring it over and see for yourself. Keep us posted.
p.s. If the OP’s neighbor is bottle-feeding the kitten, they need to “potty” the kitten. In short, newborn kittens are so immature, they don’t urinate or defecate on their own, so their rear ends need to be rubbed with something rough, like a paper towel, before they’re fed.