Most Masses with music in our 21st century Catholic environment feature a musical model best described as the “Four-Hymn Sandwich”. It consists in replacing the singing of the Proper Antiphons of a specific Mass by hymns, as in the table below. The texts of the Proper are biblical and specific to the liturgy of each Mass. The text of hymns are more general, devotional in nature, but not direct quotes from the Bible, and can be “recycled” over many different Masses.
The “Four Hymn Sandwich” was actually invented before the implementation of Vatican II, in the mid-1960s, as a “back door” to introduce some vernacular in the Latin Mass. Per the rubrics of Latin Mass, this could be done only in a “Low Mass”, not in a “Sung Mass” or “High Mass” (see rules of the Tridentine Low Mass here: Music-for-low-mass).
Then it just became the “new normal” as parishes were looking for a “shortcut” to foster the “active participation” promoted by Vatican II in the early 1970s. Vernacular singing that was already there before the council was an easy path. Since the 70’s was also a decade of wholesale “musical experimentation” in the broader culture, the actual music recommendation of the Vatican II council never had a fighting chance.
So the four-hymn sandwich model that many believe is the “Vatican II music model” is really a left-over “back door” from the Latin Low Mass turned “shortcut”. It is somewhat insulting to Vatican II Fathers to assume they would come up with such a clumsy model.
A positive description of the Four-Hymn Sandwich model could be “organic growth” inspired by the Holy Spirit. Now with over 50 years of hindsight, can we say this “organic growth” worked? Was it the work of the Holy Spirit, or were other forces at play? Attendance to Mass among Catholics has dropped dramatically in the US since the Vatican II council. And the Catholics who still go to Mass don’t sing. This four-hymn sandwich model has certainly contributed to this dismal failure. Stop digging. Now.
Now would be a good time to actually read the liturgy documents of Vatican II and start implementing them.