4.2 is a very normal reading. What sort of blood test was it, a fasting one? How long have you had the symptoms? If you are still worried, do go back to your GP, if the symptoms don't go away.
Also are you drinking enough? Not saying you aren't, but I've heard so many people complain that they are so thirsty, and when asked how much they drink say 'a whole 2 glasses of water a day' or something equally silly. And sleeping enough? Same problem with that too 'I'm so tired all the time' 'well how much do you sleep' 'a whole six hours a night' etc.