This citation from NLM is really useful.
Bugnini’s own Consilium in 1969 offered the following instruction, consistent with the Vatican II emphasis on chant over vernacular hymnody. As printed in 1 Notitiae, 5 (1969), p. 406
That rule [permitting vernacular hymns] has been superseded. What must be sung is the Mass, its ordinary and proper, not “something,” no matter how consistent, that is imposed on the Mass. Because the liturgical service is one, it has only one countenance, one motif, one voice, the voice of the church. To continue to replace the texts of the Mass being celebrated with motets that are reverent and devout, yet out of keeping with the Mass of the day amounts to continuing an unacceptable ambiguity: it is to cheat the people. Liturgical song involves not mere melody, but words, text, thought, and the sentiments that the poetry and music contain. Thus texts must be those of the Mass, not others, and singing means singing the Mass not just singing during Mass.
Sacred music is not an add on to the liturgy. It is liturgy. Sacred music is pars integrans in the sacred liturgy, that is, an integral part or, better integrating part of the whole of liturgical worship.
Thus, music for Mass must be sacred and it must be artistic. The texts sung must be sacred and relevant to the liturgy of the day. It must be composed and performed in the best manner possible, a truly artistic way.
Pastors and church musicians should… must… rethink the "four hymn sandwich".