The fourth season of the American television medical drama Grey's Anatomy, commenced airing in the United States on September 27, 2007, and concluded on May 22, 2008.

The season continues the story of a group of surgeons and their mentors in the fictional Seattle Grace Hospital, describing their professional lives and the way they affect the personal background of each character.

In addition to the regular 17 episodes, a clip-show narrated by the editors of People recapped previous events of the show and made the transition from Grey's Anatomy to Private Practice, a spin-off focusing on Dr. Addison Montgomery and aired on September 19, 2007,[1] before the season premiere.

[25] The character Dr. Erica Hahn, portrayed by Brooke Smith joined the main cast, reprising her antagonizing role in the season's fifth episode.

Upon her return, she makes Sandra Oh's character, Cristina Yang work harder for her success in Cardiothoracics and initially served as a new love-interest for Mark Sloan, played by Eric Dane.

[26] Former Dawson's Creek star Joshua Jackson was scheduled to make his return to television in a multi-episode arc as a doctor with his first appearance in the season's eleventh episode.

[27] Jackson's appearance was cancelled due to the 2007–2008 Writers Guild of America strike, and the storyline of the character he was supposed to play never aired on the show.

The majority of the show's episodes are narrated by Ellen Pompeo, who portrayed protagonist Dr. Meredith Grey, a surgical resident whose storylines are the series' focal-points.

James Pickens, Jr. portrayed attending physician and general surgeon Dr. Richard Webber, who continues his position as Chief of Surgery, despite his former wishes of retirement.

Orthopedic surgeon and fifth-year resident Dr. Calliope "Callie" Torres, who was portrayed by Sara Ramirez, has to face her husband's unfaithfulness and her unexpected bisexuality.

Seth Green of Buffy the Vampire Slayer guest starred in 2 episodes, whereas Lauren Stamile portrayed nurse Rose, a love-interest for Derek.

[29] Former regular Kate Walsh appeared for the first time since her departure on May 1, 2008, receiving a special guest-star billing in the role of Addison Montgomery, now the main character of the spin-off Private Practice.

Jeff Perry, Loretta Devine and Debra Monk reprised their roles as Thatcher Grey, Adele Webber and Louise O'Malley, respectively.

[32] On January 30, 2007, a source told People magazine that Washington was scheduled to return to the Grey's Anatomy set as early on that Thursday for the first time since entering "executive counseling" after making the comments at the Golden Globes.

His publicist, Howard Bragman, told The Hollywood Reporter that "they have the rights of the character to advance the story, but not the image" and stated he expected this to result in a "financial settlement", but it is still uncertain whether this ultimately happened.

[44] "Crash Into Me: Part 1" was outperformed in the time slot by CSI: Crime Scene Investigation's "You Kill Me", the Thanksgiving special episode which attracted 14.75 million viewers and received a 5.2 rating, ranking #11 in the week.

Debbie Chang of BuddyTV.com expressed disappointment in the shows' development throughout the season, by stating it was "all about couples, jumping in-and-out of relationships, trying their darndest to have hot sex on the cramped, twin-sized bunk-beds in the on-call room.

"[75] However, the episode "Physical Attraction, Chemical Reaction" received a positive review, with Burrows stating that it "fully encompassed all the things that make this show great: intense emotional drama and macabre OR activities.

"[75] Jack Florey of IndieLondon reviewed the characters, stating that their behaviour is the show's biggest problem: "the self-absorbed, pretentious and frequently-selfish attitudes that drive the surgeons at the centre of Grey’s exasperate more than reward.

"[76] As for the storylines, Foley stated that they didn't "ring true" and that "the plot devices became increasingly clunky", noting the lack of realism in arcs such as George and Callie's marriage and the Izzie's affair "as a means of ripping it apart.

"[76] Florrey also commented on Meredith Grey's arc, by stating that she turned into "one of the most selfish, self-centered characters on television", whereas Mark Sloan's storyline was named "sex-obsessed, borderline-misogynistic and close to scandalous.

Katherine Heigl who portrayed Izzie Stevens declined to put her name forward for consideration at the Emmy Awards, claiming that she had been given insufficient material on the series to warrant a nomination.

"[81] Sara Ramirez's portrayal of Callie Torres was positively reviewed, resulting in the actor receiving a nomination at the 2008 American Latino Media Arts Awards.

Under the title Grey's Anatomy: Season Four – Expanded, the box-set consists of episodes with Dolby Digital 5.1 surround sound and widescreen format.