The decision to end this after 4 series was a very wise one; not because it was beginning to shows signs of wear and wear but because the natural dramatic arc of the characters was able to be satisfying concluded. And while a number of characters had closure (Aimee, in particular, was particularly powerful and moving) others had come to terms with themselves and those whom they loved and if not having reached total closure, were at least on, some very well on, the road to happiness and self-understanding. The final episode provided a very satisfying and cathartic conclusion.
And by the end, I had realized that Otis was not the only therapist (ostensibly only, although in actuality not, about sexual matters) but everyone, at different times and in different ways had acted as/were therapists, either for others or for themselves. The art and skill of this was listening and emotional empathy. A marvelous concept to take away at the end of the series.