Destiny Hawk

Just like Rouge One or Solo, Star Wars 'stories'; Destiny Hawk is not a 'book 4' exactly, but in essence, a CG spinoff. It deals with a brand new character, one not previously introduced in the trilogy. Her name is Abbi, and she is - Cassie's half-sister, the daughter of Sandra and David - years after the events of the Cross Over.
And just as with her sister Cassie and mother Sandra (the dual persona of Cassandra), Abbi's full name, AbbiGale, is her duality designation - with her winged, sea - creature, borne of the methane lakes of Saturn's moon, Titan. Unlike the merged personhoods of previous human-to-human characters in the Cross Gate world, Destiny Hawk explores the two-as-one of a girl and the animal she's bonded with.
More is happening upon 'the new Earth' as well, as the planet's restored sentinels (Eve & Adam) are in crisis. Eve has been kidnapped by none other than Viator, the Beast returned: Both gone back in time to Eden via a wicked scheme he's concocted to reboot the timeline into original sin. Shepherd, aware of the situation, has a grand design of his own for Eve's rescue, spearheaded by Abbi & Gale, with ground support from the rest of the gate-keepers, not to mention the four living creatures of Revelations. It proves to be as epic in its conclusion as its predecessors!