4 year old female always had low vitamin b12 had high infection 4 weeks ago now has platelet count well above average white blood cell count increased?

Check again: Often blood cell abnormalities come and go for a variety of reasons. I would recommend you see your pediatrician or family doctor and have your child thoroughly examined. A normal exam will be very reassuring. I would also repeat the blood tests in a week or two and compare. I would also talk to the doctor about diet and vitamins for the B12 deficiency.