I always thought that, if they weren't some kind of "Lost Tribe-esque" Revan/Malak branch, they were probably a part of Nihilus faction. Sion never cared for building a military army (his forces on Peragus, Korriban and Malachor are nearly all assassins or lesser Sith students), while Nihilus is known for having a massive fleet and an army capable of planetary scale invasion, so most probably some Lord under Nihilus command wanted to investigate a planet as resourceful in fuel and minerals as M4-78. I know this is added by the modders, but the dead Sith Lord near the entrance of the Environmental Zone supports this, as he is wearing the robes of Nihilus' Sith.
BTW, they are all remains of Revan/Malak Empire, at first they were all a united remnant under Traya's leadership, with its hideout in Malachor, but when Sion and Nihilus betrayed Traya, the Triumvirate splitted, and it seems Nihilus took the larger part of the army while Sion kept Malachor.