No4=D91M

no4=D91M (called on the JASRAC database) is Disc 1, Track 4 on the XenobladeX Original Soundtrack (making it the fourth track in 0rCH-SUITE "X"). X-BT4 is a version of this track without percussion, and PianoX1 and the first part of aBOreSSs are both remixes of this track.

It plays in the following cutscenes:
 * Lao explains his motivations between the phases of the Prog Ares fight in.
 * Yelv accepts the fate of his partner during Yelv's Partner.
 * The deceased Auld Belgazus's body is given to the Auravis during A New Life.
 * HT665: Erio tells Cross where her sisters' base is after destroying Fortun during Definian Downfall.

Etymology
The no4= at the start of the track indicates that this is the 4th track on 0rCH-SUITE "X". Moreover:
 * The note D is also known as 'Re' in the Fixed Do Solfege music education method.
 * The number 9 is pronounced 'kyu' or 'ku' in Japanese.
 * The number 1 looks like a capital I in English. Alternatively, 1 in Japanese is 'ichi', which can be shortened to 'i'.

Putting these together (and ignoring the 'no4=' at the start), the track's name decodes to 'Re-ku-i-em', or 'Requiem'. This aligns with its JASRAC title.