The Borg Class 4 tactical vessel was a type of heavily-armed Borg cube utilized by the Borg Collective. The tactical cube differed from the standard Borg cube in several ways, the most notable being the extensive hull armor that covered a large portion of the exterior hull of the ship. The interior was also slightly altered from what is known of Borg "architecture". The vessel's central plexus was protected by multi-regenerative security fields.
Only one such vessel was observed by Starfleet – by the USS Voyager in 2376. The tactical cube was caused to self-destruct by the Borg Queen during a brief civil war within the Collective. (VOY: "Unimatrix Zero", "Unimatrix Zero, Part II")
Ships of the class
The Borg tactical cube was designed by Doug Drexler at the request of the producers who wanted some ideas pitched for a new Borg cube design. Drexler incorporated in his design the notion that the Borg performed tactical updates after their recent beating by Species 8472, or as he put it, "What do you do with a cube? You can armor it. The Borg had just had their collective asses handed to them by Species 8472, so I'd design a flak jacket for a Borg cube. Personally I felt adding anything to the cube that gave it personality was a mistake. I happened to love the idea that the design said absolutely nothing about them, while at the same time, everything."  The model was built at Foundation Imaging by Koji Kuramura, who used the texture maps of the standard Borg cube as a basis, they themselves being mapped from the physical studio model used in Star Trek: First Contact.  As Kuramura explains, "I started with the base Borg cube model. And added the parts that Doug had on his drawings. I don't know if many people know this, but there was never more than 3 sides built for the Borg cube, because if you think about it... You could never see more than that on screen. The reason being was that it was a very Polygon heavy model. And back in those days of Voyager, memory was at a premium."  At the time, Drexler was also posted at Foundation Imaging as VFX coordinator, so Kuramura was able to confer with him about details as he was building the model.
During the design phase, Drexler shortly toyed with the notion of a pyramid-shaped Borg ship. He commented, "My one joke was the Borg pyramid. I mean think about it. First we had the Borg cube, then the sphere from "First Contact", and now the pyramid! It's the UPN logo!" 
- The Borg tactical cube was also used in the climactic final level of Star Trek: Legacy. Oddly, creators of the game boasted in a video interview that the vessel was of their creation, and that it had been designed especially for the game's finale. This is highly unlikely, as, not only had it clearly first appeared on Star Trek Voyager two years before, but Doug Drexler's name was not mentioned at all during the interview in question. Also, when the vessel appeared during the mission, Admiral Janeway says she had encountered such a vessel before, and had help destroying it, clearly referencing the events of the episode.
- Tactical cubes are also featured in Star Trek: Armada II, in which eight of them can combine to form a massive tactical fusion cube.
- Ships of this class also feature in Star Trek: Conquest; however, the ship is ranked as a "cruiser" and has inferior fire power and shielding to an average cube.
- Tactical cubes are found in several campaign missions of Star Trek Online, classified as a dreadnought. Each one has approximately four times the weaponry of a standard cube and roughly ten times the hit points, requiring a small fleet to destroy.