AT: "xx" SC-4 was the designation of a Starfleet Command shuttlecraft used by Admiral Kathryn Janeway of an alternate 2404 in her plan to alter history. Equipping the shuttle with a chrono deflector, Janeway generated a temporal rift to the Delta Quadrant of the year 2378, where she successfully infiltrated the Borg Unicomplex and delivered a neurolytic pathogen to cripple the Collective. This allowed USS Voyager to use a Borg transwarp conduit to return to the Alpha Quadrant sixteen years before they would have without Admiral Janeway's intervention.
The shuttle was capable of achieving the speed of warp 6, and it was equipped with deflector shields and several advanced 25th century technologies, including a stealth generator, an ablative armor generator, transphasic torpedoes, and a neural interface and synaptic transceiver. All of which Admiral Janeway shared with the crew of the Voyager in 2378. (VOY: "Endgame")