Determining fixed dates and circa dates of births gets a bit dicey. The profile for Ermengarde shows a fixed birth year of 970, and this profile for her mother shows a fixed birth year of 930. Father and husband Guillaume IV's circa birth year is 945, From what FMG suggests, [Guy], who's virtually a shoe-in as the of Guillaume IV and Humberge, would be the last-born child.
In looking at the most recent FMG update of 20 March 2022, the FMG site gives a rationale that Humberge / Ermengarde is most likely the one whose name is hyperlinked. When clicking on it, FMG shows an estimated birth of [970/75], If one accepts this uncertain dating, then the ca. birth year for Guillaume IV would put him ca. 25 years younger than Humberge / Ermengarde.
http://fmg.ac/Projects/MedLands/AUVERGNE.htm#GuillaumeIVAuvergnedie...
http://fmg.ac/Projects/MedLands/toulnoreast.htm#HumbergeErmengardeB...
