Long term relationships do get back together after breakups. But in my opinion there is normally another realtionship in between. Think about it, after 4 years + with the same person, and it didnt work out. You would certainly want to meet someone else, even if just to make a comparison to check what you just lost was normal, or infact better! So they normally take longer before they reconcile or even get back in contact.
I have some friends who were high school sweethearts, never thought they would break up. Ultimately they did after a few years (3 I think) anyway they lost contact with one another and eventually both met some one new each. Both of these failed, and they then both found another new partner. Again they failed. after a couple of years they managed to locate each other and have been together ever since. So about...13 years! kids, married the lot and happy!
I also know some other friends of the family that were broke up early twenties....they found each other when they were early 40's and started things again. I think they are both mid early 50's now so its been atleast 10 years. Fair enough it was a huge time gap of almost 20 years but in the end they found each other again.... and claim they have never been happier. Life works in mysterious ways!