Brother/sister, look at 4:20 from the point of veiw you understood from 4:1/2/3/4 now, since these were never about allowing multiple marriages. It is simply saying if you have decided to marry another woman then don't take away any thing you have given in form of gifts and other things from the one you're going to leave now! And don't use any such tactic to degrade and hence force her into giving up anything too.
About 24:3, we have discussed this earlier on previous form. It's about people(males and females) who keep on having adulterous way of life, they should be together, but its not about those who repent and work hard to mend their ways.