Mark IV

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