I don't think that the season four narrative was set up perfectly; parts of it were certainly a little awkward and rushed. I do, however, think that the redefinition of the typical narrative signposts was part of the point, so it doesn't seem right to me to criticize the show for ultimately upending them: the entire series was about reconsidering the usual tropes of romantic comedy. Ending the show with Rebecca choosing a romantic partner would have been an absurd betrayal of that entire premise.
I have to add that, even though the finale wasn't perfect (how many finales are?), parts of it just made me think that Rachel Bloom is a genius. The scene with Rebecca walking through the costumes of the series and singing an unaccompanied medley of some of the lines from the show's songs was just haunting. I also loved the meta ending, which ultimately boils down to the human need for creativity, to the way that artists are able to turn the broken bits of themselves into beauty. Rebecca's ultimate lesson was that creativity is love, and that art is the path to healing herself. That's awesome.