Romans 4:6-7 — Even as David also describeth the blessedness of the man, unto whom God imputeth righteousness without works,
Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered.
David is used in addition to Abraham because Jewish law required two witnesses and because Abraham was before the Law and David was under the Law. Faith was without works for both, but in the Old Testament, sins were covered (guilt removed) not removed.
Blessedness — not just happiness, by a pronouncement by God.
Verse 7 is a quote from Psalm 32, after David was forgiven for his sin with Bathsheba. Blessed is he whose transgression is forgiven, whose sin is covered (Psalm 32:1).