I took in these three 4 week old kittens and last night one of them got sick, but I was able to keep the kitten alive and now all three of them are happy and healthy. They all play, eat regularly (kitten formula and sometimes soft food). I am keeping them warm. I took them to a vet and have been dewormed and checked for parasites. Are there any other tips or advice anyone can give me so they can stay happy and healthy? So far so good, but anyone have any extra advice?

Not really advice, but have you considered contacting your local shelter/vet/etc. and seeing if they can put you in touch with someone who has done rescue work with cats? They would have the most practical, tested advice that I can think of. Explain your situation, and say that you are looking for someone to act as a mentor, and they may be able to find someone to advise you.