At the end of Season 3 all of House's fellows left his practice, either quit or were fired by him. Most of Season 4 featured House recruiting new fellows via a impromptu 'reality show' like set up whittling down a couple dozen candidates to four (Chris Taub, Remmy "Thirteen" Hadley, Laurence Kutner, and Amber "Cut Throat Bitch" Volakis.) He ended up firing CTB and hiring the other three, CTB was fired because she wouldn't easily admit to being wrong.
CTB eventually got romantically involved with Wilson much to House's chagrin. In the penultimate episode of the season House is in a bus crash and is suffering memory loss but is aware that someone he saw is near death, it's eventually discovered this person is Amber.
In the final episode of the season we learn that she answered a call for Wilson, from House, to pick him up at a bar. Wilson was on call at the hospital so Amber went to pick-up House to take him home; House refused and boarded a bus followed by Amber and both were in the crash.
Amber sent to another hospital as a Jane Doe, House takes charge of her "case" (she suffering from kidney failure) and we find out that Amber was overdosing on anti-virals for the flu. Her kidney failure meant that they weren't properly filtering out the drug and the drug bonds with certain proteins in the body, without the kidneys it wasn't being filtered out and the drug was causing damage to all of her internal organs. When her hear stopped on an ambulance en-route to the hospital House works at (from the hospital she was originally in) nothing could be done to save her. (In the ambulance her heart stopped but Wilson and House decided not to restart it rather to put her into "protective hypothermia" to prevent whatever was killing her to be pumped through her body to cause more damage, at the hospital she was kept in hypothermia and put on bypass.)
In short, Amber was dead. She was awoken briefly from a medically induced coma to say goodbye to Wilson and friends (House himself now in a coma due to a risky memory-recalling procedure he went through to solve the case.)
Her death tore Wilson up and strained the friendship with House as House was, partly, "responsible" for her death (had House not been so dependent on others to bail him out of trouble Amber would've never gone to pick up House or followed him onto the bus.)
You'd have to watch the whole season to really "get" it namely the relationship Amber had with House and the one Amber and Wilson had that formed a bit of a "love triangle" with Amber coming between House and Wilson's friendship. Her death was very well handled, shocking, and we even get a bit of a humanity insight into House. Also? Cuddy does a strip-tease showing us all how good looking a middle-aged woman really can look.
If there's one season I'd recommend seeing of House's it's Season 4, the shortest season due to a writer's strike. It's just very compact, tight, and I don't think there's a bad episode in the bunch, it also introduced me to the perfect beauty of Olivia Wilde. The show really hasn't topped those two hours.
