Where does it hurt? Did you do something strenuous to pull a muscle? How active are you? Did you have an x-ray or other scans done? When did you see your doctor last?
If you saw a doctor 4 years ago and still have the pain then go back to a doctor. If you are sedentary, you might want to do some exercises to strengthen your back as your muscles help hold your spine in place so every time you lift something your back doesn't go out. If you had a scan or x-ray, what did it say? Sometimes, people have test done but the doctor's office forgets to get the results.
The severity of the pain should tell you something. If it is severe, you would have gone to a doctor or ER by now. If it is intermittent, when do you get the pain? What do you take for the pain? If Tylenol or Advil helps, you can take these when you are having pain. Write down every thing about your pain and take your list to the doctor. She/he will want to know more about when, what and where. Since you continue to have pain, some type of diagnostic test is in order: x-ray, CT scan or MRI. Usually the x-ray will be first and then if it shows something, a CT scan or MRI may be ordered.