Normal white blood cell values range from 6000 - 19000/L. Even ranges as low as 4900 have been considered normal. You may see this as 6.0 x1000/L .
Here is some additional information for you:
The WBC can be low when there is viral illness, bone marrow suppression, or isolated infection causing sepsis where a large portion the wbc isolate to this one particular area. I would definitely consider more testing and abdominal xrays. It would be a good idea to determine why her numbers are so low. She does need to be on antibiotics as a preventive to protect her from developing infection. I would also contact your vet about getting her started on Viralys (L-Lysine) which is an amino acid supplement that boosts the immune system.
FIV is Feline Immunodeficiency Virus or Feline AIDS. This was one of the tests your vet has likely performed. She should be on antibiotics as a preventive. I would also suggest Viralys in combination with the vitamin supplement. If your cat seems to continue to get worse then she should have further testing to determine the cause. If you recently got her from a shelter then feline distemper is also a possibility.
Panleukopenia in cats:
Please let me know if further assistance is needed.