A pain that you have had for four years is not likely to be cancer; however, you do need to see the doctor and find out what might be causing the problem.  It's not unusual for one breast to be larger than the other, but your 30 pound analogy sounds like the difference is larger than usual.  I'm not a doctor so I can't say what the best diagnostic test would be.  If the pain and size difference have been stable for four years, then make an appointment to see the doctor, but there is no rush.  If you have had a recent change in the pain level and swelling, then try to get the appointment as soon as possible.  If you are running a fever or if you have redness and skin dimpling on the swollen side, then call the doctor today and ask for an immediate appointment.