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 Chapter 11 when Lao explains his motivations between the phases of the Prog Ares fight.

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.

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.