Big 4: Fed, Rafa, Murray, Djokovic
Second tier: JMDP, Vulture, Bird, Stan, Tsonga
2.5 tier: Healthy Gasquet
Third tier: Haas, Raonic, Almagro, Nishikori, perhaps Isner
This is about right.
Also aying someone is in the big 4 doesn't necessarily mean they are among the four best right now or a contender for slams. It just means they are one of Federer, Nadal, Djokovic, Murray. The Big 4 was a name that started being used for those particular players because they continuously occupied the top 4 rankings slots, won nearly every big event (slams, masters, WTFs, Olympics) and occupied a large number of semifinal and final spots too, from the period from around 2008 and particularly 2010-2012. Even when they retire you could still call them the Big 4, because the name itself refers to those four players. It's not a default that there will be a "current Big 4" of whoever the best four players of the moment are. If some other combination of players go on to dominate the tour over the next few years the way that Federer, Nadal, Djokovic, Murray did then maybe it'd make sense to call them a "next Big 4", but the best four players in the world at a given moment don't automatically equal "the big 4".
There has been a top tier of 8 players for a while, but now Wawrinka has officially broken into that. Hard to say for sure yet if any of the other 8 are on their way out yet, like Ferrer or Tsonga. With in those 8/9 players, it's hard to come up with a "tier" of a clear top 4 and clear "next four" right now, as many things are up in the air with some players rising, some falling, some coming back from injury etc.