You must force feed her. At 4 weeks she can lick up cat or baby food.
Cats are attracted to smelly foods - the stronger the better. If she wants tuna, give it to her You can also try fancy feast, clam juice or baby food.
human baby food in meat flavours (check that there are no onions or garlic in the ingredients) and mix that with warm water and offer that, or syringe it in little bits into your cat's mouth. Beech Nut makes a line of baby food that has nothing but meat (beef, chicken, turkey or veal) in it. Here's a link: http://www.beechnut.com/Our%20Baby%20Food/Results.asp If you cannot find this, you could find another meat baby food - just read the label carefully to be sure there are no onions, onion powder, garlic, or garlic powder in it.
If her tummy needs a rest, feed baby food with no onion or garlic for a few days.
If she wont eat, begin by giving her two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When she feels hydrated she will lick up the gruel on her own.
Give her droppers of unflavored pedialyte a few times a day to make sure she stays hydrated.
If she throws it up have her seen immediately!
Please let me know how she is doing. I will always be here for you.
I hope she feels better very soon.
Sincerest best wishes,