SC-4 was the designation of a Type 18H shuttlecraft assigned to a future Starfleet Commandd used by Admiral Kathryn Janeway of an alternate 2404 in her plan to alter history. Equipping the shuttle with a chrono deflector stolen from Korath, Janeway generated a temporal rift to the Delta Quadrant of the year 2377, where she successfully infiltrated the Borg Unicomplex and delivered a neurolytic pathogen to cripple the Collective. This created a new timeline in which allowed USS Voyager to use a Borg transwarp conduit to return to the Alpha Quadrant seventeen years before they would have without Admiral Janeway's intervention.
The shuttle was capable of achieving the speed of at least warp 6. It was equipped with deflector shields and several advanced anti-Borg technologies developed by the crew of Voyager in the alternate timeline before reaching Earth in 2394, including stealth technology, an ablative armor generator, transphasic torpedoes, as well as a neural interface and synaptic transceiver, all of which Admiral Janeway shared with the crew of Voyager in 2377. SC-4 was presumably destroyed along with the rest of the Borg Unicomplex.
This type of shuttle was similar to the type 9 shuttlecraft of the 24th century, but more streamlined.