There is no difficulty for Catholics. We see no conflict between our faith and Romans 3 & 4 when we read it. Our understanding of the two chapters is as follows:
Chapter 3 starts with a continuation of the argument about circumcision begun at 2:25. Paul's argument culminates with the conclusion at verses 19-20 translated by the Jerusalem Bible as follows:
19 Now we are well aware that whatever the Law says is said for those
who are subject to the Law, so that every mouth may be silenced, and
the whole world brought under the judgement of God. 20 So then, no human
being can be found upright at the tribunal of God by keeping the Law; all
that the Law does is to tell us what is sinful.
No arguments from Catholics in this regard.
The question specifically asks about verses 21-22:
21 God's saving justice was witnessed by the Law and the Prophets, but
now it has been revealed altogether apart from law: 22 God's saving
justice given through faith in Jesus Christ to all who believe.
Catholics have no problem at all with this sentence. It is foundational to our faith that the justice of God comes to us through our faith in the merits of Jesus Christ, specifically earned on the cross. The law could only reveal God's justice, it could not empower us to live it in any way. The thought is further clarified in the rest of the paragraph:
23 No distinction is made: all have sinned and lack God's glory, 24
and all are justified by the free gift of his grace through being set
free in Christ Jesus. 25 God appointed him as a sacrifice for
reconciliation, through faith, by the shedding of his blood, and so
showed his justness; first for the past, when sins went unpunished
because he held his hand; 26 and now again for the present age, to
show how he is just and justifies everyone who has faith in Jesus.
The next verse inquired of is 3:28, which is not a complete sentence in the Jerusalem Bible. Starting at verse 27 then:
27 So what becomes of our boasts? There is no room for them. On what
principle- that only actions count? No; that faith is what counts, 28
since, as we see it, a person is justified by faith and not by doing
what the Law tells him to do.
Note that Paul here is not dismissing all actions. The context is the works of the Jewish law versus faith. Catholics insist that we who are not of the circumcision are not bound by the Jewish law. Furthermore, the gospel makes available salvation by faith even to those of the circumcision since it is impossible to live in righteousness by the law, because the law provides no grace for keeping it. We are saved by faith, just as Paul teaches here. We take our theology directly from these very verses.
However, note that Paul does not say "faith alone", but rather "faith". Catholics understand faith to be a lifestyle, not a decision, not merely a once-and-for-all-times assent to abstract truths. The only Biblical citation to "faith alone" is James 2:17 where it is condemned: "Even so faith, if it hath not works, is dead, being alone." (KJV) Living faith, then, must result in good works just as apple trees result in apples. It's not the good works that save, but the merits of Christ. Nevertheless, the good works must follow if you are to have any personal assurance of salvation.
The question also specifically asks about verses 4:4-6. I prefer to treat the entire paragraph as a unit, but I will bold 4-6:
1 Then what do we say about Abraham, the ancestor from whom we are
descended physically? 2 If Abraham had been justified because of what
he had done, then he would have had something to boast about. But not
before God: 3 does not scripture say: Abraham put his faith in God and
this was reckoned to him as uprightness? 4 Now, when someone works,
the wages for this are not considered as a favour but as due; 5
however, when someone, without working, puts faith in the one who
justifies the godless, it is this faith that is reckoned as
uprightness. 6 David, too, says the same: he calls someone blessed if
God attributes uprightness to that person, apart from any action
undertaken: 7 How blessed are those whose offence is forgiven, whose sin is
blotted out. 8 How blessed are those to whom the Lord imputes no guilt.
Now a quick review of the life of Abraham reveals that he had many actions which proved his faith in God. His was a living faith. He packed up all his belongings and left his homeland not knowing where he was going. He circumcised himself and all his household as an old man. He believed God unswervingly for years first for a son, and then a son to be the father of many nations. He proved himself ready to sacrifice his son Isaac as a holocaust, knowing that it was Isaac that was to produce the many nations. Hebrews chapter 11 goes into Abraham's faith in detail.
Yet for Catholics, all good works come as a result of grace, specifically sanctifying grace, so no wages are ever due for them. All is grace (favour in the above citation.) A Catholic would say that Abraham received much grace in order to believe as powerfully as he did. Works are the proper fruits of righteousness (uprightness in the above citation); they are never the reason why God imputes righteousness.
Coming back to the concept of assurance of salvation, verse 11 says in part "circumcision was given to him [Abraham] later, as a sign and a guarantee that the faith which he had while still uncircumcised was reckoned to him as uprightness." Thus, we see in action the Catholic doctrine that good works, while not in themselves salvific, do give us assurance that our faith is operative, and therefore we can have assurance of our salvation.
In verse 16 Paul makes the point about faith coming by grace (free gift):
16 That is why the promise is to faith, so that it comes as a free
gift and is secure for all the descendants, not only those who rely on
the Law but all those others who rely on the faith of Abraham, the
ancestor of us all
The chapter ends at verse 25 with the statement:
we believe in him who raised from the dead our Lord Jesus who was
handed over to death for our sins and raised to life for our
Again, no arguments from Catholics. We believe every word of it.