JM4 (Knight/Professional/Captain) is the eighth rank in the Dark Brotherhood, which is awarded after completing the requirements of a JM3 (Hunter/Padawan/Yeoman/Lieutenant). At this Rank members may construct and wield a Lightsaber from the Lightsaber Creation Interface.
It is the last rank and culmination of the Journeymen class.
Sub-Unit-Quaestors have the authority to promote up to this rank.
The Requirements to be promoted past this rank are very different compared to previous Journeymen ranks, and it is determined by your Clan Summit when a promotion is warranted.
The requirements for the promotion from JM4 are found at the above link, the completion of the above requirements will earn a promotion to Equite 1.
Upon promotion by your Clan Summit, you will reach one of five Equite ranks based on your Order.
Sith Order: Warrior
Dark/Gray Jedi Order: Mystic
Jedi Order: Peacekeeper
Loyalist Order: Major
Mercenary Order: Privateer