A good friend of mine was (is) in a similar situation. Her boyfriend of a year is going to university across the country and a week after he left he called her to break up with her.
The catch is that when he came home for Thanksgiving and they saw one another, they ended up in bed together. Following this they made the decision to get back together.
I am good friends with both of them and having talked to both, I've realized that she is clinging to the familiarity of her boyfriend and the relationship and that he feels it's easier to stay with her, because, as he put it, she's a "sure thing." Easiness and familiarity are not good reasons to stay in a relationship. Just as relationships cannot be based solely on sex.
Is it really that good when you're together? Or is it maybe that you're both so comfortable with one another? If things aren't good when you're apart, but good when you're together, chances are that things may not actually be so wonderful in all. For a relationship to work, I believe you also have to be happy when you're apart.
But, listen, you two have had the staying power of FOUR YEARS! I mean, that's impressive. Very. Maybe it would be a good idea to take some time off to be single and try the dating scene and try new things. Then perhaps later in life if you two should meet up again the sparks might fly?
Hope this helps!
Smile, though your heart is breaking....
My crazy little universe....