Mark IV was a type of TARDIS, far more advanced than the Doctor's TARDIS.

The Monk's TARDIS was a Mark IV, which he stole from Gallifrey when he became a renegade Time Lord fifty years after the Doctor left. A Mark IV TARDIS had a smoother materialisation action compared to the earlier Type 40. (AUDIO: The Resurrection of Mars)

A Mark IV TARDIS had a directional unit that was far more powerful than the directional unit of the Type 40 TARDIS. The First Doctor stated that the increased energy rate would destroy the Time Rotor if one was added to the console. (TV: The Daleks Masterplan)

It also had an automatic drift control which suspended the TARDIS in space with absolute safety. (PROSE: The Time Meddler)