How low?
The normal range for white cells in an adult is around about 4.5-10.0 x 10^9 / litre. This is for 95% of the population, therefore 1 in 20 people can be expected to fall outside this range and still be normal.
A count of 3.9 may mean nothing, a count of <1.0 definitely does.
There is a huge number of reasons for variation in white cell count, e.g. merely running for a bus will send your count up. A low count? Again many many reasons, viral infection is one, but it looks as though nobody is panicking which is reasssuring.