K9 Mark IV

The Doctor created K-9 Mark IV as a replacement for the destroyed K-9 Mark III.

Biography
After Mark III sacrified himself to defeat the Krillitanes, the Doctor left a new model, Mark IV, for Sarah Jane Smith, just seconds after he left in the TARDIS. (DW: School Reunion)


 * Though not directly stated, we can assume that K-9 Mark IV contains the recorded memories of K-9 Mark III.

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 wristwatch that could detect alien life (Sarah Jane Smith website).

K-9 Mark IV left Sarah Jane in order to stabilize a dangerous artificial black hole created in Switzerland, though located in space. He spent a year and a half in this task and did not know when he would complete it. He could only speak with Sarah occasionally, when the orbit of the black hole intersected with that of Earth. (SJA: Invasion of the Bane)

Features and improvements
This K-9 had a shiny metallic blue paint job and a number of improvements over Mark III including an undetectable hyperlink facility. (DW: School Reunion) Later, though, K-9 reverted to his old, dark green color. (SJA: Invasion of the Bane)

Behind the Scenes
The designation K-9 Mark IV never actually appeared in dialogue, but we can assume it.