Kerber is a tremendous player and if he body holds up will be among the top 4 in the next 5 years, but she is too physically spent to expect her to beat the likes of Halep and Pliskova again at this year end tournament.
Radswanska will not have enough to win again playing shots against power.
Muguruza is my second top 4 players in the next 5 years but she is playing shits after Roland Garros win.