Is Paul still talking about justification in Romans 4?
Yes. Paul has not left the subject of justification behind; he continues dealing with it in Romans 4. To clarify it, he uses Abraham from the Old Testament as an ideal example of a man who was justified, showing that justification meant the same thing then as it does today.
Les explains that in Romans 4 Paul isn't leaving that word justification behind yet. He is still dealing with his whole business of justification. To clarify it, Paul comes back to an Old Testament character, Abraham, whom most Bible people have heard of and know the circumstances. Abraham becomes an ideal example of a man who was justified, and Les notes that being justified means the same thing back then as it does today.