Dating is what people do before entering into a relationship—or it's what most people used to do—and you two are already in a relationship. It was a FWB relationship, yes, but it was still a relationship. And people in relationships don't typically go out on dates. So, yeah, the reason going out on a date with your boyfriend feels awkward is because you're not dating, LID, not at this stage. You're together. So be together: Go places, do things, have dinner, see friends, go home, sex amazingly. Spend more time together, build on what you've already established (i.e., the emotional and sexual connection that carried you through the last four years), and stop stressing about performing the roles of "boyfriend" and "girlfriend."