I wasn't certain either, anymore. I knew what I felt when we were together but apart I wasn't feeling it as much. Absence doesn't always make the heart grow fonder.
I think that you need to sit down and talk to her face to face. Has she drifted away or maybe she doesn't love you like you love her. It has been four years and a lot of changes may have occurred with her.
PS. Remember that line from Good Will Hunting -- "I have to go see about a girl"