None of them are even close to calling it quits, there is absolutely no one that can challenge them. Maybe in the next few years we see a few more Del Potro-esque runs but I see these four dominating the majority of the slams at least for the next 5 years. There is just absolutely no one out there capable of becoming a top player.
I am a fan of Federer, Nadal, Djokovic and Murray. Yes, that's possible.