The T4-B heavy tank was a multi-purpose tank used by the Rebel Alliance in the Empire at War, as well as its Forces of Corruption add-on. When you reach Tier 3, a group of 3 can be constructed in the Rebel Heavy Factory for 1100 credits.
It has a very slow build time, but it will be worth the wait, as each tank has second-best armour in the game and possesses powerful anti-vehicle laser cannons. Using its special ability will instantly switch it to the concussion missile launchers: this gives it increased range and vastly improved area-of-effect damage against infantry and buildings: 3 T4-Bs can wipe out a full-health Stormtrooper Platoon or a structure like Shield Generator in two volleys when using missile launchers. The missiles will also bypass the shielding on 2-M Repulsor Tanks and unupgraded Empire Shield Generators.
Its main drawback is speed; these tanks are the slowest unit in the game. As such, out of the multitude of upgrades available for this unit, the two Tracks upgrades should come first. Its slow speed makes it a good idea to pair the unit with R2-D2 and C-3PO, as the duo can then easily follow the unit without falling behind and can repair heavily damaged tanks if the need arises. This lack of speed makes them a great target for SPMA-T Artillery. In short, T4-B squadron is well-equipped to defeat just about any enemy it encounters, from the Stormtroopers to the 2-M Repulsor Tanks and even AT-ATs. The SPMA-T Artillery can make a reasonable counter to this unit, while Mara Jade and Darth Vader can destroy lone tanks with Thermal Detonators and Force Crush respectively. The best option, however, is to use the Palpatine's Force Corrupt on them: it will likely affect the whole unit, turning this ludicrously powerful vehicle against its creators.