You know, four years is a long time to just up and toss like this.
And to resolve things with someone, you don't need to re-establish a romantic relationship with them. In fact, that's a really crummy idea, especially since a breakup after four years isn't going to leave you in ANY place to be with someone new right away.
But you can simply TALK to that ex: have lunch or coffee. bring up whatever you feel is unresolved. This is generally how one seeks resolution maturely and soundly.
bear in mind, too, that realy does anything with anyone feel 100% "finished." People are just more complex than that, as are relationships.