Although the same size as a regular cube the class IV carries heavy tactical armour and is armed to the teeth.

As seen in Voyager this is the Borg Queen ship (otherwise known as the Diamond)

Dominator by Captain Fingers

Dominator by The Great One

This is an unconverted 3ds model of the Class 4 "Tactical Cube" as featured in Star Trek Voyager Episode: 'Unimatrix Zero'

Fighter by JkerPlay

Monolith by Captain Fingers

This is a special edition release of a TNG "canon" Borg vessel which was refered to as the "Transwarp Prototype" as used by the Renegade Borg in the double episode called "Descent Pt1/2". Although this model is not 100% accurate to the original version shown in the episode it is around 97% correct.

Derived as a part of the borgs growing Tactical part of the collective this ship (typically called a "H" because of its shape), is specifically design to fire a massive volley of Borg heavy torpedos at an enemy.

Heavily armoured, and armed to the teeth the Tactical Pyramid is somewhat smaller than the cube, but just as deadly.