SC-4

From Memory Alpha, the free Star Trek reference.

SC-4 was the designation of a shuttlecraft assigned to a future Starfleet Command.

As part of Janeway's plan to alter the timeline, and bring USS Voyager back to Earth twenty years early, it was equipped with a chrono deflector, stolen from Korath.

The shuttlecraft traveled back to 2378, and was used by Admiral Janeway to infiltrate the Unicomplex and deliver a neurolytic pathogen into the Borg Collective. (VOY: "Endgame")

In 2404 of an alternate timeline, this shuttlecraft was Admiral Kathryn Janeway's personal vessel. The shuttle had been equipped with a stealth generator, ablative hull armor, transphasic torpedoes, and a neural interface and synaptic transceiver system.

The shuttle was presumed lost and possibly assimilated by the Borg after it was detected by the Borg Queen. although considering the fact that it was in Unicomplex 1 at the time and that complex was destroyed, it was probably destroyed as well. Furthermore, since Admiral Janeway succeeded in sending Voyager home before 23 years had elapsed, it can be assumed that she herself, and the shuttle never exsisted as Voyager was back in Federation space, before said 23 years would have elapsed, nullifying the reasons she went back in time in the first place.