EP David Shore says that a storyline will kick off in late April that will build toward the series finale on May 21:
“The final four episodes are, to some extent, [part] of a piece. But the last one is very, very much a much separate culmination of those episodes.”
Shore is unwilling to share any specifics yet on the plot of the finale, but he calls the last hour a "fitting ending" in which "a few" of the show's alumni will make a reappearance (Olivia Wilde being one).
Shore also hopes that the show will not be judged solely on the last episode but on the quality of the series as a whole:
“[Series finales] get put under such a microscope. And I sort of resent that in a way. I understand that people are going to be paying attention to it, but… we will have done 177 episodes. I would like to think every single one of them speaks for the series. I don’t think this one should speak much louder than the rest of them.”
Source: TV Line