Why do II only sleep 3 to 4 hours a night? Ii don't drink a lot of caffiene

Good question. There are so many reasons why people don't sleep well. One has to look at work habits, how active one is, how stressful one's life is & one's health status is. Not to be ignored is possible depression, anxiety or sleep apnea. A workup by a primary care physician can help answer why you can't sleep well.
Depends. This depends on if you are feeling tired during the day. There are genetically short sleepers. Otherwise, if you feel you could use more sleep but aren't getting it, see a sleep medicine specialist for some recommendations that fit with your life and medical issues. Make sure it is not related to something else.