third and fourth definition
third and fourth is an idiom that means “whatever number” (cf. the idiom “for three, even for four” in Proverbs 30:18, 21, 29 and in Amos 1:3, 6, 11, 13, etc.) This is in contrast to the loyal love that God has for “thousands of generations.” The numerical disparity drives home the basic point: mercy triumphs over judgment (James 2:13).