There have been four seasons of Grey's. Here's a recap of everything that went on.
***This is taken from the guide in the Season 4 DVD.***

• Meredith, Crisina, Izzie, George, and Alex all start their first day as intens at SGH with Dr. Miranda Bailey, aka "The Nazi," as their resident.
• Meredith discovers last night's one-night stand is her new boss: Dreamy neurosurgeon Dr. Derek Shepherd.
• Derek and Burke begin to vie for the Chief's position.
• Meredith and Derek have their first "real" date: breakfast at an outdoor French cafe.
• Meredith allows Izzie and George to move into her house. Meanwhile, George seems to be developing a secret crush on Meredith.
• Burke and Cristina have their first on-call room encounter.
• Izzie throws a party at the house, where Bailey learns of Meredith and Derek's relationship.
• Richard undergoes Neuro surgery, to remove a tumor pressing on his optic nerve. Upon awakening, he sees Derek comforting Meredith.
• Addison Forbes Montgomery Shepherd arrives at Seattle Grace.
• Cristina tells Meredith she's pregnant. She refuses to tell the father, Burke, until it's too late...Cristina eventually suffers from an ectopic pregnancy and loses the baby.
• Ellis is admitted to SGH and Meredith's secret is out: Everyone learns of Ellis' disease.
• George becomes "the heart in the elevator guy," after performing heart surgery in an elevator during a hospital wide power outage.
• Derek and Addison work on heir marriage by attending couples counseling.
• Alex kisses Izzie in front of everyone at Joe's Bar and the two begin a short-lived romance. The relationship ends when Alex is caught cheating with "George's skanky syph nurse," Olivia.
• It's Thanksgiving, so George joins his father and brothers on the annual O'Malley Turkey Hunt.
• Izzie meets handsome heart paitent Denny Duquette. The two seem to share an undeniable, immediate connection.
• "Code Black!" Richard shuts the entire surgical wing of the hospital as Meredith finds herself in a rather serious, potentially deadly predicament. George helps deliver Bailey's first child, Tuck (William George Bailey Jones)
• After an ill fated night with Meredith, George meets orthopedic resident Dr. Callie Torres.
• Meredith declares a vow of celibacy and takes up knitting, then meets her dog's cute vet, Dr. Finn Dandridge ("McVet").
• Burke is shot. His life and career remain in Derek's hands.
• Denny proposes to Izzie. Izzie is left heartbroken after deciding to take Denny's situation into her own hands. Sadly, Denny dies... And Izzie quits the program.
• At the SGH prom, Derek and Meredith find themselves alone in a patient room. After a heated argument,they finally face their true feelings for each other.
• Richard begins to visit Ellis in her nursing home, bringing the past back to the surface. Meanwhile, Adele, Richard's wife, gives her husband an ultimatum: either the hospital or her.
• Izzie refuses to face her peers at work, so she stays far away from Seattle Grace. She continues to grieve the death of Denny by baking muffins.
• Derek finds Addison in a hotel room... with Mark Sloan. "McSteamy" becomes Seattle Grace's newest attending.
• Meredith decides to date both McDreamy and McVet.
• Unbeknown to anyone, Cristina begins covering Burke's trembling hand during their surgeries.
• Wills are tested and discoveries are made when all of the boys venture deep into the Seattle wilderness on a camping trip.
• Guilt-ridden and sick of hiding his tremor, Burke goes to Richard's office to confess. However, Burke is quite surprised to find Cristina already there, apparently telling the Chief everything.
• George's father is admitted to the hospital with cancer.
• Addison and Alex share a kiss at Joe's.
• Izzie finally decides what to do with her $8.7 million inheritance... The Denny Duquette Memorial Clinic is born.
• George and Burke each suprise their girlfriends with marriage proposals. Callie immediately accepts, whereas Cristina takes it "under advisement."
• A major ferry boat accident occurs in Seattle, sending all of our doctors into action.
• Derek rescues Meredith after she falls into the Sound, and Alex saves a Jane Doe patient who he will later name " Ava."
• Meredith flatlines and visits some of SGH's most popular (albeit dead) patients in what seems to be some kind of afterlife. At the same time, Ellis dies.
• In the wake of Ellis' death Meredith's father, Thatcher, attempts to rekindle a strained relationship with his daughter. Thatcher's wife, Susan, also attempts to forge a relationship with Meredith as well.
• Izzie and George take their friendship to an entirely new level and find themselves struggling with new found feeling.
• The interns take their residency exams. Unfortunately, George fails.
• Derek meets a girl at Joe's. Little does he ( or anyone else) know that it's actually Lexie Grey, Meredith's- half sister.
• Cristina and Burke's wedding day comes... and quickly goes...
• Richard offers Chief to Derek, who turns it down. According to Derek, Richard is still the best man for the job.
• Meredith, Cristina, Izzie, and Alex begin a new year as residents, and George begins his second first day as an intern.
• Meredith tries to avoid her half-sister at all costs. Lexie simply isn't a girl Meredith wants to get to know.
• Callie's work begins to suffer as she becomes increasingly suspicious of George and Izzie.
• Derek and Mark put the past behind them and seemingly return to their old friendship.
• Much to Cristina's dismay, Erica Hahn becomes "the new Burke."
• Mark struggles to mend his manwhore ways and become a "man of substance."
• After an emotionally charged day, Derek shares a special moment wiht a scrub nurse named Rose.
• Bailey's baby, Tuck, is involved in a serious accident at home.
• Could therapy be just what the doctor ordered for Meredith?
• Ava (real name Rebecca) returns to SGH with some starting news for Alex. Izzie, however, continues to believe that something is amiss with the former Jane Doe.
• Richard attempts to make amends with Adele by finally learning to delegate at the hospital.
• Rumors abound as Callie starts to form an unlikely relationship with Erica Hahn.
• In the hopes of making medical history, Meredith and Derek begin to work together on a series of clinical trials. Could this be the catalyst for the long awaited Mer-Der reunion?