Characters & Cast
The Doctor: David Tennant
The 10th version of the Doctor, he begins Season 4 companionless again. He meets Donna Noble again, and they go off in the TARDIS together. The Doctor is reluctant to bring on another companion; after Rose's painful goodbye, and Martha's sudden departure, the Doctor doesn't want another companion. But Donna manages to convince him he needs someone traveling with him. This time around, the Doctor is much more fun and laid back, and he and Donna are a fantastic team. The Doctor has a lot of big changes in his life during this series. In "The Doctor's Daughter", a young girl is generated from the Doctor's DNA, creating a daughter. While the Doctor insists she's not his daughter, they still bond, and the Doctor accepts her as his. He also meets River Song, who has a connection to his future self, though he doesn't know her in the current series.
Donna Noble: Catherine Tate
We first meet Donna in the Season 3 Christmas Special, "The Runaway Bride". She mysteriously appears in the Doctor's TARDIS. She's a bit annoying when we first met her, but after seeing what the Doctor does, she decides she's going to live a better life. But she refuses to join the Doctor on his travels. When we meet her again in Season 4, Donna has regretted not accepting the Doctor's offer, and has spent the last year or so looking for him. She's still living at home with her mother (her father passed away), and her grandfather Wilf. She happily joins the Doctor on his travels, and helps him to settle down a bit. She also keeps him grounded, and not as reckless as he had been. She refers to herself as a "Supertemp!", but it's quite obvious that she's really brilliant.
Recuring Cast/Characters
Wilfred Mott: Bernard Cribbins
(Appears in Voyage of the Damned, 7 Other Episodes)
Donna's grandfather and closest friend (besides the Doctor). He loves stargazing, and is the only other member of the Noble family who believes in aliens. He helps out the Doctor and Donna when they're on Earth, though mostly he frets about Donna. But he's pretty much awesome. He and Donna have a really wonderful bond, and he's also really fun. Wilf is devestated by the end of "Journey's End", but promises to look out for the Doctor.
Martha Jones: Freema Agyeman
(Appears in 4.04, 4.05, 4.06, 4.12 and 4.13)
The Doctor's companion in Season 3, Martha returns in "The Sontaran Stratagem". She's now a full fledge Doctor, and working for UNIT. She calls the Doctor to Earth to help UNIT investigate ATMOS. Martha seems to be over her crush on the Doctor, and is now engaged to Tom Milligan. She is cloned by the Sontarans in 4.04, but is freed by the Doctor in 4.05. At the end of "The Poison Sky", she is saying goodbye to the Doctor when the TARDIS starts to go off. She is dragged along with the Doctor and Donna. Martha returns home at the end of 4.06, but comes back to the series for the finale. She uses "Project Indigo", which is an experimental transport, to try to find the Doctor. However, she is transported home, and instead connects with the rest of the Doctor's companions to call him to Earth. At the end of "Journey's End", Jack invites her to come work for Torchwood.
Rose Tyler: Billie Piper
(Appears in 4.01, 4.05, 4.10, 4.11, 4.12, 4.13)
The Doctor's companion in Season 1 and 2, Rose returns in the fourth series. She first appears in "Partners in Crime", when Donna runs up to her. Rose then turns, walks away, and disappears. She is seen again in "The Poison Sky" and "Midnight" on TV screens, screaming soundlessly for the Doctor. Rose finally "speaks" in "Turn Left", when she meets Donna in an alternate universe, where Donna never met the Doctor. Rose is mysterious, and never tells Donna her name. But she pushes Donna along and tells her bits and pieces about what's really going on. She helps Donna right the universe, and before alternate!Donna dies, she tells her two words; Bad Wolf. In "The Stolen Earth", she appears in London, and finds Donna's family to get to the Doctor. She and the Doctor reunite, but he is shot by a Dalek. In "Journey's End", she along with the rest of the Doctor's companions stop Davros and the Daleks. However, the Doctor brings her back home to the parallel universe, asking her to look after the human!Doctor. The parallel universe is finally sealed off for good.
Captain Jack Harkness: John Barrowman
(Appears in 4.12 and 4.13)
Captain Jack returns in series four! He, along with the rest of Torchwood (Gwen Cooper and Ianto Jones), work with the rest of the Doctor's companions to try to get the Doctor to Earth. When the Doctor finally returns, he goes to the TARDIS to help him. While on board the Dalek's ship, he escapes and finds Mickey, Jackie and Sarah Jane. In "Journey's End", he offers Martha a place in Torchwood.
Sarah Jane Smith: Elisabeth Sladen
(Appears in 4.12 and 4.13)
The Doctor's companion from the original series, Sarah Jane returns in the two part series finale. She helps the Doctor's companions contact the Doctor, then with Mickey and Jackie, manage to get onboard the Dalek ship. Sarah Jane is much happier when she leaves the Doctor this time.
Mickey Smith: Noel Clarke
(Appears in 4.13)
Mickey returns from the parallel universe with Jackie. Together with Sarah Jane and Jackie, they get into the Dalek ship to help the Doctor. He chooses to stay behind though, and doesn't return to the parallel universe. He walks off with Jack and Martha, implying he will join the Torchwood in this universe.