Quartum penitus ignoro cum sint tria

As the rubric highlights, this antiphon, in melodic style, is a "demonstratio modi", with the purpose of bringing to mind the main features of the melodic coutour. The first word of the latin text chosen recalls the number of their tonal group. In this case, the text is from Proverbs 30: 18 "Tria sunt difficilia mihi et quartum penitus", but in reverse order, starting with "Quartum". It is worth noting that the melodic contour is higher than usual in the 4th mode, touching the c.