The Starfleet shuttle flown by Admiral Kathryn Janeway in 2404, fitted with ablative armour and a stolen Klingon time device, which she used to go back and bring Voyager home early.
SC-4 is a small Starfleet shuttle of the early 25th century. It appears in Endgame, the final episode of Voyager.
In a future where Voyager took twenty-three years to get home, Admiral Janeway resolves to change history. She takes SC-4, which carries deployable ablative armour and other advanced systems, and obtains a chrono deflector from the Klingon Korath. Pursued by Harry Kim's USS Rhode Island, she opens a rift and emerges beside Voyager in the Delta Quadrant in 2378.
The shuttle's technology is copied to arm Voyager against the Borg. The admiral then flies SC-4 into the Unicomplex and lets the Borg Queen assimilate her, carrying the pathogen that destroys the transwarp hub.