The normal adult range for a total white count is 4500-10,000. If yours was, say, 4300, that would be slightly low and nothing to be worried about. Slightly low is not the same as a critical low. If this had been the case, the lab would have phoned your doctor to report this. All critical lows must be phoned to the doctor. Except for feeling blue, you say you feel fine. Depression won't decrease your white count.
Now some people here are talking about anemia. It's possible that you have a mild anemia. However, you should know that there is more than one kind of anemia. Most people think that iron-deficiency anemia is the only type and that iron supplements are the answer to all anemias. They would be wrong. Iron-deficiency anemia concerns the red cells, not the white. Decreased levels in white cells can be due to aplastic anemia and pernicious anemia. These two affect the red cells along with the white. There are other types of anemias as well. And there are other conditions that decrease the white count but they may not apply to you.
To give you peace of mind, contact your doctor and ak if there's anything that she/he recommends at this time. And if you need a little help with the blues, try talking to a friend. It may help.
I'm a nurse.