A couple of years ago I had a CBC that came back a little off. My wbc was a little low, around 3.9. 21 year old male. Could I have leukemia? Or would I have had worse symptoms by now?

Not by this number. Leukemia is a specific process where the WBC numbers actually are tremendously high, not low. That happens in weeks to months not a few years & is accompanied by other symptoms. It is not unusual to have intrusive thoughts about health problems at your age. If these are interfering with your life get a good checkup & talk with your doc. Those thoughts can cover-up other areas you need to work on.