NV4 (Proselyte/Trainee/Trooper), is the fourth rank in the Dark Brotherhood, which is awarded after completing the requirements of a NV3 (Novice/Recruit/Cadet).
It is part of the Journeyman class, the fourth step down the path to Knight.
Battleteam Leaders have the authority to promote up to this rank.
The requirements for the promotion from NV4 are found at the above link, the completion of the above requirements will earn a promotion to JM1 (Neophyte/Runner/Private).