4* players can also perform good and help the team, but you can't expect much from a 4* team.
I 've never met a 4* team in semi finals of a Cup or a CH.L. or winning the league, but it was teams which had some useful 4* players among the others.
As I read your previous post about an injury and the lack of red packs, I insist not to buy a young, expensive, fast trainer. Without red & greens you can get the added value of such a player. So try to find again a good, 5*, 30 player. If he has some good previous stats, it's a good sign, but doesn't secure that he 'll continue playing good in your team too