30 y.o female. CBC shows WBC of 4.0. Range for lab was 4.4-10.8. Flagged as low. All other labs incl. Diff were normal. Last year WBC 4.17. Concern?

Any symptoms? All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Talk to the doctor who ordered the tests. Having said that, a stable count around 4 may be your normal and not to be concerned about.
Wish you good health!