Angel 2.0... what say you about running my fleet with this ship?. Its been retrofitted with better, deadly and larger guns.
—Bells to Angel 2.0 upon looking at the Schnee's Justice
Imperial IV-class Star Destroyer was Bells' variant of the Imperial II-class Star Destroyers that was modified by Nakamura Industries to change its original turbolasers to those she wanted installed onto the starship. Unlike the Imperial II Star Destroyers, hers weren't outfitted with heavier armour plating than the original models and instead the armor plating was reinforced by the heavy shielding generators from Nakamura Industries.
Schnee's Justice was one of the first Star Destroyers to be built and it served as Bells' primary flagship with ISS Valkyrie being her secondary. Angel 2.0 was placed as Supreme Commander of the Nakamura Military and used ISS Valkyrie as his own flagship but did not take full ownership over it as it still was Bells' personal Imperial II-class Star Destroyer.
Although much like the Imperial II-class Star Destroyer, the Imperial IV-class was only modified to have its weapon systems be replaced with ones that Bella had of her choosing upon getting the Nakamura workers to install them on the ships.
The older, regular shielding on them were replaced with heavy shielding generators to maximize its power against laser projectiles while solid projectiles, missiles and mass drivers, still go right through and penetrates the armor.
During its first production above Kuat's shipyards, Bells found a new mineral that she used to modifiy the damage output of the turbolaser and ion cannons. The name of the new mineral, she couldn't find out so she called it Dust and inputted it into most of the weapon systems on the Star Destroyers.
The armour plating wasn't tempered with as Bells wanted space for more guns to fit into the ship which made it stronger than Imperial II-class Star Destroyers. It shields were also fitted with the mineral Dust and made it stronger but since the armour plating was the same as Imperial I-class Star Destroyers, the ship's survival without shields would depend on its starfighter support which it carried not many TIE starfighters.
It also only had one docking bay for the guns that Bella wanted and with that, the ship only carried a half wing of TIE Fighters, TIE/INs and TIE Defenders due to the less space they had for them to dock with. It also limited the landings of officer shuttles as well as hers which bothered her a lot but didn't want to remove the extra guns.
The Imperial IV-class Star Destroyer was first used during the Battle of Sullust where Bells helped Grand Admiral Thrawn in his campaign against the New Republic but instead of fighting against the New Republic, Thrawn was tracking and trying to take down Varanee and her bounty hunter, Eclipse for interferring with his missions and interests. Bells successfully took out Varanee but lost the bounty hunter and searched for her for weeks. She later gave up looking for the Twi-lek as she continued to operate as leader for the Nakamura Alignment.
It wasn't known on what replaced the Imperial IV-class Star Destroyers as of Anna's death. Preceeding imperial governments weren't able to get their hands on the blueprints for her variant of the Imperial II-class Star Destroyer nor the mineral only known to her as Dust.