Hi, For the past few weeks I've had headaches, dry skin and mouth, frequent thirist and urination, I'm drinking more than 4liters of water a day?

See a doctor. Something is not right, as you know. It is hard to say what it could be without knowing more about you, and your history. See a doc as soon as possible and get a thorough history and physical done with appropriate lab work. Stop any herbs or supplements you may be taking, as well as drugs and alcohol for now. Good luck!
See doctor. To be checked for diabetes. Best of health:)