+++Warning, this contains minor spoilers from season three, but no major season four spoilers+++
By the fourth season, people will really know if they love Psych or not. It is really a show that you are going to (probably) love or hate. I do not think there are a ton of people out there that are lukewarm to the show. It is about a guy named Shawn Spencer (James Roday) with extremely high observational skills who has convinced the Santa Barbara Police that he is actually psychic, and works as a consultant for them along with his best friend Gus (Dule Hill). If you combine a sitcom with a prime-time police procedural, throwing in a ton of 80s and 90s pop culture references, you would get Psych. This season continues to mostly be a case-of-the-week procedural, but there are tiebacks to prior episodes, most notably the Yin-Yang killer, with Ally Sheedy reprising her role as Mr. Yang, and Jimmi Simpson reprising his role as Mary. And we get closer to the totally pop-culture-themed episodes that the series would become known for, with a Halloween episode that paid homage to the 1930s and 1940s monster movies, and an episode that starred Ray Wise that was a total spoof on his Twin Peaks character.
For those who get the DVD set, there are a ton of bonus features that are very similar to what was included in the seasons 1-3 released. There are deleted scenes for most episodes, commentaries for select episodes, a gag reel, and several montages (usually of Shawn or Gus saying the same thing over and over). So, if you like watching the bonus content, then you get a lot (which is becoming rarer and rarer as time goes by).
Overall, the season is very good. The show continues to get a ton of great guest stars including Rachael Leigh Cook recurring in her role as Abigail Lytar, Kurt Fuller who made his Psych debut as Woody the Coroner and would recur for the rest of the series run, Ally Sheedy, Jimmi Simpson, Christopher Turner, Cary Elwes as art thief Pierre Desperaux (who would also recur through the series), Ray Wise, Jaleel White, Christine Baranski, Jim Beaver, Josh Braaten, James Brolin, John Cena, Jay Chandrasekhar, Tim Conlon, Miguel Ferrer, Azita Ghanizada, Stacy Keibler, Joshua Malina, David Naughton, Judd Nelson, Larisa Oleynik, Robert Patrick, Sendhil Ramamurthy, Lisa Ray, Michael Rooker, Jeri Ryan, Sarah Shahi, Kenan Thompson, Tony Todd, Beverley Turner, Arnold Vosloo, and Thomas F. Wilson. With the most recognizable guest stars, the show usually makes at least one or two jokes that referred to the character and/or show/movie that made the person famous. So, if you liked the first few seasons, you will most likely feel the same way about this one. It is definitely worth watching.