Routine fbc shows low white blood cell count (3.9 10*9/l) and neutrophil count (1.4). everything else normal. doctor not worried. what can cause this?

Many causes: These are relatively minimal abnormalities, but should not be dismissed without some follow-up over time. Low white and neutrophil counts can be caused by some drugs, autoimmune disorders, infection or a bone marrow disease. Occasionally this may be a totally benign chronic condition. You should have another CBC in a month or so and if still low continued monitoring.

May be your normal: Your WBC count is within the normal range though at the lower end of the range. If you do not have any symptoms there is no cause for concern. See this site for more info on this topic. http://www.mayoclinic.org/tests-procedures/complete-blood-count/basics/results/PRC-20014088

Just being alive: There's enormous variation in white blood cell counts from routine blood tests. Just being alive causes 3900 WBCs and 1400 neutrophils, meaning lots of people have these numbers. Numbers are just numbers. So, without the whole story, nothing interesting can be said or concluded about these two numbers.