K9 Mark IV

"There was a scientific experiment in Switzerland, they created their own black hole. If it gets free the Earth will be swallowed up. So K-9 is sealing it off."

- Sarah Jane Smith

K-9 Mark IV served as a replacement for the destroyed K-9 Mark III.

Biography
"Why does he look so... disco?"

- Rose Tyler on K-9 Mark III

After Mark III sacrificed himself to defeat the Krillitanes, the Doctor left a new model, Mark IV, for Sarah Jane Smith, just seconds after he departed in his TARDIS. K9 refers to himself as being rebuilt and improved, suggesting Mark IV possesses the same memory as Mark III. (DW: School Reunion) Though Sarah Jane didn't find this out right away, the Doctor had also placed some gifts in a panel in K-9's side: sonic lipstick and a Scanner Watch that could detect alien life. (Sarah Jane Smith website)

A year before Sarah Jane first met Maria Jackson, K-9 Mark IV left Sarah Jane in order to stabilize a dangerous artificial black hole

in space which had been created in Switzerland. He could only speak with Sarah occasionally via a cabinet with a spacetime link when the orbit of the black hole intersected with that of Earth and did not know when he would complete the job. (SJA: Invasion of the Bane) However, at times he was capable of returning to Earth briefly, and did so on two later occasions in order to render vital assistance. In the first time, he fought Mr Smith in pitched battle to allow Sarah to upload the Armaggedon Virus, (SJA: The Lost Boy) And again in order to quickly upload the Doctor's TARDIS' base code to Mr Smith and assist the Doctor in returning the Earth to its correct location in space. (DW: Journey's End)

After these events he went back monitoring the black hole only to find an unidentified member of the Slitheen family attempting to steal him as part of a revenge plot against Sarah Jane, who had foiled their plans in the past. K-9 banished the Slitheen, after reversing one of its attacks.(SJA: From Raxacoricofallapatorius With Love)

Sarah Jane then destroyed the black hole by transferring it into fuel for Eve's ship. K-9 then triumphantly returned to Bannerman Road, to stay. He greeted Mr Smith, who holds a grudge against him and has a photo taken with Luke Smith, Clyde Langer and Rani Chandra. In 2059, Rani still has that photo. (SJA: The Mad Woman in the Attic)

In 2056, Sarah left K-9 to a boy who would help later alongside with Drax to help search for the Sixth Doctor who was held captive by Omega. (DW: Search for the Doctor)

Features and improvements
"Oi! Listen, in the year 5000, this was cutting edge!"

- The Tenth Doctor

This K-9 had a shiny metallic blue paint job and a number of improvements over Mark III including an omniflexible hyperlink facility. (DW: School Reunion) Later, though, K-9 reverted to his old, dark green color. (SJA: Invasion of the Bane) In episode 2 of The Lost Boy Sarah Jane unleashes K-9 Mark IV to battle Mr Smith. Before and after the battle K-9 displays a teleport facility and utilises a laser weapon similar to the previous K-9s'. (SJA: The Lost Boy) Later it is shown that he has a hover mode. (The Wedding of Sarah Jane Smith)

Behind the Scenes

 * The designation K-9 Mark IV never actually appeared in dialogue. Documents packaged with the 1/4 scale Remote Control K-9 toy referred to this model as the Mark IIIb, but props in The Sarah Jane Adventures have had the Mark IV designation.
 * In the 2009 version of the Doctor Who Annual "She (Sarah Jane Smith) has not seen much of K-9 as he is busy sealing off a black hole."