My wbc count is 4.2. range: 3.8-10.8. should i keep an eye on this? mpv is 12.3. range: 7.5-12.5. anything to worry about? platelets were low prior

Depends on why tests: Numbers are just numbers, with little meaning unless one can see the big picture. Generally, a normal person with a routine screening blood count result of WBC 4.2 does not have to worry. If she currently has a medical problem that might affect her white blood cell count result, she can talk with her ordering doctor about that. Most importantly, that doctor knows why testing was done.