At Comic-Con, thousands of fans hoped to get a clue on what will happen when the fourth season starts September 19th. They saw the first 40 seconds on the season opener, but it looked uncertain if Beckett (Stana Katic) will survive. The executive producers would only say that all questions will be answered by the end of the first episode.
Fillion, though, isn’t sure if Kate and Richard should get together. He did admit, "things were said that can’t be taken back," but that doesn’t mean they’ll be dating. "I am one of the mind who thinks if they get together, the show is over," he said. "Conflict is what draws us to Castle, if you resolve the conflict I think there’s little left." He did think executive producer Andrew Marlowe would make sure the relationship between Kate and Richard will stay realistic.
He also praised Ruben Santiago-Hudson for his work on the show as Montgomery. "He’s an incredible actor, but also an incredible man," Fillion said. "The ultimate sacrifice is to step aside, and give the show…you give yourself to the show so that it might move on."

There could be other factors that may become  roadblocks in their relationship. One would be the new Captain, Victoria Gates, played by 24’s Penny Johnson Jerald. Gates will make it clear she doesn’t want Castle hanging around at police headquarters, which means new problems. "There’s a guy who doesn’t take no for an answer," he said, "not because he’s mean, but just because he’s used to doing whatever he wants to do." Gates is also expected to get some resistance from Medical Examiner Lanie Parish (Tamala Jones) and Beckett if and when she recovers.

Another problem could be Alexis, played by Molly Quinn. Her character has been used to taking care of her dad for so long, and she’s not sure about giving her dad to Beckett. "To see his attention being taken by this other woman," she says, "and she’s not really treating him in an appropriate way that I think he should be treated, I think, if anything, there will be more friction between Kate and myself."
Quinn also surprised everyone in the panel by wearing her version of the Malcom Reynolds costume her character once dismissed on the show.

The new season will also explore Parish’s romantic relationship with Esposito (Jon Huertas), and how that is affected by Detective Ryan’s (Seamus Dever) upcoming wedding.

So love and tension are in the air when Castle comes back in September. The producers also announced that a third Nikki Heat book, called Heat Rises, will be coming in the fall. There will also be a comic book "written" by Castle called "Deadly Storm" also coming this fall.