Season 3 and Season 4 of Castle are complete reversals of each other.
Castle is about a famous mystery novelist, Richard Castle, who has come to a dead end with his book series. Castle is brought into questioning by the NYPD because there was copycat murder from one of his novels. Now interested in crime and murder Castle gets new ideas for his book series. Using his connection through the mayor he is able to follow Detective Kate Beckett. Castle then models his new character in his book series, Nikki Heat, after Beckett. Beckett initially disproves of Castle’s presence when working on cases, but she then sees him as a useful resource.
In season three of Castle, Beckett has a new boyfriend Josh Davidson. Castle is speechless when Josh dropped off Beckett at the precinct because she did not tell anyone she was dating. Throughout season 3 Castle struggles to understand his feelings for Beckett. Since the relationship between Beckett and Josh is getting serious Castle realizes that he must tell Beckett his true feeling for her. Before he can do so Beckett is severely shot and he holds her while paramedics are called. Afraid that she might not make it, Castle confesses his love for Beckett before she passes out.
In season 4, Beckett survives and tells Castle she does not remember anything from after she was shot. She then breaks up with Josh during her recovery. Castle is surprised by this information, but does not want to pressure Beckett into a relationship while she is recovering. A little later Castle overhears Beckett while interrogating a suspect that she remembers everything that happened the day she was shot. Feeling betrayed Castle starts pursuing other women. Now Beckett struggles with her feeling for Castle while he starts seeing other women.
Season 3 and season 4 are reversals because the person in a relationship change and the person struggling with their feelings switch. It first starts with Beckett in a relationship, then changes to Castle. Castle struggles with his feelings first in season 3 and Beckett then struggles with hers in season 4.