Just make sure it gets plenty of milk from the mom. At 4 weeks of age you can go ahead and start it on wet dog food. Something that would be great for the dog is Nutrical ( vitamin ). You can get it from the vet. It costs $6 for a tube where I live and that tube goes a long way.
I agree with most of what Judyann had to offer you,but,as a breeder of Chis,I know they are getting their teeth in by 3 weeks old.I start my babies out on a pet replacement formula,mixed with some baby rice cereal.Real thin at first,then thicken it up.At 3 weeks,I sit on the floor with them and pick them up one at a time and get some on my finger and get them to taste it.They usually take to it pretty well.When all that is accomplished,by 4-5 weeks they can begin working on Pedigree Lamb and Rice...canned food for puppies.Lamb & Rice is best because Chis can be allergic to a number of things,and beef is one of them.Then by around 6 weeks old,they are always nosing around picking at their parents dry food,so you need to supply them with a decent,SMALL bites puppy chow.The larger chunks can get stuck in their tiny throats and cause death.
Keep very close eye on them feed them canned food cooked in the microwive for about 5 to 10 secs and feed and water them every 3 or 4 hours don't leave the water out they can drown. Do this until they get about 2 months old and help them get on dry food then.
By feeding dem puppy chow or warm milk at least 2 times a day!
I just recently got a newborn chihuahua & is just 5 weeks. Did not know that they should be older? Do I feed her milk or how to take care of her at such a young age?
How do you keep the pups from getting to cold I put a blanket in with the pups and mom and they are in a 74 degree house will they be ok?
What happened to Mamma? Ideally, if there's no way the pup(s) could be with mom you would see your vet so they can show you the correct way to do everything since it's so easy to make mistakes with young pups. You'll need puppy replacement formula, little bottles or even hand feeding syringes if those aren't available. The formula can't be too hot or cold. You have to make the hole in the bottle small or too much liquid too soon and the pup might inhale it and aspirate. You can't feed on the back or in many positions or they can inhale it and aspirate (this usually results in death). Each feeding you need to try and 'stimulate' the puppy to go potty with some warm water on a cotton ball until they are regularly going on their own. Since this is a Chihuahua, make sure you have some nutristat or nutrical around (time for that very soon and maybe even a tiny amount mixed in but have the vet show you that) and that you don't miss any feedings. Based on the size of a 'runt' Chihuahua you're probably still at around every 3 hours or maybe less (the bigger they get the longer they can go :-). You'll have to watch very close. You have to make sure the pup is warm enough too. You should probably be able to start the weaning process (slowly) in a week or two depending on the puppy itself. It really is best to have your vet guide you through all this as mistakes are easily made and you can call them whenever you are concerned or worried or have a question about something.
I just saw your shout from Chi. She clearly has more of the story (details make all the difference sometimes). I'd listen and she used to do Chi's in particular too. I'd follow her advice. Based on what she said it's probably the only thing you can do and it says momma stopped caring for the pup. That usually indicates a problem with the pup that needs to be seen by the vet. Nature dictates you don't waste resources on a defective or sick one when the healthy ones could use it (however, maybe it's as simple as clearing up the fleas that may have made the pup anemic or something or special eating instructions and a bit of help for awhile, you won't know unless you ask). Thakfully we have veterinarians that usually correct the problem or let us know when to give up and let them go and don't have to rely on nature. Good luck and let us know what happens.
You let the mom handle it
Just let mum do the work now and start offering wet food at about 3 weeks old.