Has the mom had quads before?
If she does not have enough milk for all, she will often abandon the weakest one so that the others may have a better chance to live.
You might want to get some goat's milk and a glass eye dropper. The eye droppers (I usually have at least two on hand) may be purchased from most pharmacies. Goats milk is usually found with the canned milk or baby formulas. It's a bit expensive, but worth every penny. Once opened, store in a glass airtight container in the refrigerator or in a plastic container in the freezer. The milk needs to be room temperature. Do not warm in the microwave ... the microwave kills all the good stuff in the goat's milk.
Videos on how to feed can be found here:
http://luvnchins.com/ChinsIISignsTrouble.html It may take a while for them to load, but you can click on the arrow to replay it. Make sure you feed the largest kits first ... this gives the smaller/weaker ones a chance at more of momma chin's natural chinchilla milk. I usually hang a small bottle of goat's milk on the cage also (about 2 ounces at a time) ... if momma drinks some of it ... that's OK ... it's good for her, too.
Jo Ann