Usefulness of 4*s
I don't think 4*s have outlived their usefulness. 4*s are important for a number of reasons, mainly growth. But we all constantly hear about the game shifting to 5*s, even though it is difficult for people outside of the top alliances to get them, let alone dupe them. But for even the majority of alliances in the expert tier we are told 5*s are more important. The reason for this is because prestige sets the difficulty of your map in Alliance Quests which then impacts your total points you can earn. As long as a rank 4 level 55 unduped 5* has a higher prestige than a max ranked 99 sig 4* people are going to say 4*s are no longer useful. While I don't personally agree with that thought, they become less useful to those that are in the top alliances. I'm not saying it is a problem either. It's just the current state of things in the game and that's what drives people's thoughts and reasons for what they do in the game.