After the slightly disappointing season 3 grey's anatomy season 4 had to be the season that was best by all standards, and i have to say it even out performed the unmissable season 2.
During the season we watched the now residents relationships flourish and diminish. The biggest mistake ever was Izzie and George getting together although the break up of George's marriage opened the floodgates for a relationship with a difference for Callie. Throughout the season viewers were waiting in anticipation for the MERDER realtionship to make a move forward... although this looked very doubtful after Derek starts "dating"
Cristina gets a shock from her past, and is challenged in her chosen career path in Cardio-thoracics upon the arrival of Dr. Erica Hahn. This season shows the abrput, comedy and competitive nature that cristina is well known and loved for, although we also see her more vulnerable side, when the shock appears and especially towards the season finale when it's obvious that Cristina doesn't want to loose her "person" and best friend Meredith-( this continues into season 5)
There are a few guest appearences this season, as well as new characters who join the regular cast, one in particular Lexie Grey - Meredith's half sister.
Season 4 contains all the usual medical drama, comedy and romance we have become accustommed to, In my opinion is the best season to date, definately one that should not be missed!
Enjoy