Just got labs back WBC low at 3.7 but rbc, hgb, & hct were all wnl. Second time in 4 months its been slightly low. Should I be worried?

Need evaluation. You should not be worried yet at this time. But, you should definitely talk to your doctor. There are many things that can cause low white blood cell count- from drugs, infection, autoimmune problems, congenital, vitamin deficiency, malignancy etc. Your doctor should do further evaluation. If needed, your doc can refer you to a hematologist ( blood specialist).
NOT REALLY. Borderline low WBC count is very non specific, and even perfectly healthy people may run on the low side. However, in the event that the patient is ill, elderly or immunocompromised, receiving chemotherapy or on some rheumatological or psychiatric drugs, the WBC count needs closely monitored.