Who does David say is blessed or happy in Romans 4?
David describes as blessed or happy the man unto whom God imputes righteousness without works—apart from any doing on man's part. God puts righteousness to that man's account as a gift, not as something earned or merited by works.
Les explains that David is describing the blessedness, or happiness, of the man unto whom God imputes righteousness without works. God imputes righteousness without our doing a thing. Les contrasts this with Christian good works and connects it to Ephesians 2:8-10: salvation is by grace through faith, not of yourselves, not anything we can do. It is the gift of God, and you do not work for or merit a gift; if you did, it would no longer be a gift. That is what salvation is.