As to Count IV.
IV. Conscience can truly and rightly judge that sexual acts between persons who have contracted a civil marriage with each other, although one or both of them is sacramentally married to another person, can sometimes be morally right, or requested or even commanded by God.
– Open Letter to Bishops of the Catholic Church, Easter Week, 2019
In FUNDAMENTALS OF CATHOLIC DOGMA | VII. THE SACRAMENT OF MATRIMONY, the couple De fide dogmas[i] are not relevant to this count.
What is adultery?
CCC 2380 Adultery refers to marital infidelity. When two partners, of whom at least one is married to another party, have sexual relations – even transient ones – they commit adultery. Christ condemns even adultery of mere desire. The sixth commandment and the New Testament forbid adultery absolutely. The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry.
With this definition, Count IV can, therefore, read:
IV. Conscience can truly and rightly judge that [adultery], can sometimes be morally right, or requested or even commanded by God.
Conscience can truly judge right what is wrong?!
Therefore, Count IV. is a proposition that is nonsensical, contradictory, erroneous and wrong [as opposed to right], false [as opposed to true], and an intrinsic impossibility.
Marriage is a true and proper Sacrament instituted by God. [De fide.]
The Sacrament of Matrimony bestows sanctifying grace of the contracting parties. [De fide.]