Murray's level is higher than either Nadal or Federer at the moment, also Wawrinka, despite playing unbelievably in this tournament, will likely shit the draw for another 6months, though if he turns up for slams like he does then I guess it doens't matter how he fares in the smaller tournaments.
As for the career achievements, the "big 4", if ever existed, remains the same: Federer, Nadal, Djokovic, Murray. Stan, despite I like him, has to add something to his CV to surpass Murray...
As for the actual results or form, Nadal arguably does not belong here. Federer is disputable (I don't want to judge according to RG results only). My opinion is...the best 4 players at this moment are Djokovic, Murray, Wawrinka and...most likely Federer.
Um, if you're going to remove one of the four and replace it with Wawrinka that should probably be Nadal rather than Murray, based on the fact that this year Murray has lost out only to Djokovic and is only behind him and Wawrinka in results, while Nadal is flirting with dropping from the top 10... Djokovic, Federer, Murray and Wawrinka are in fact the current top 4, so that should probably have been your argument.
Anyway "the big 4" were just something from a couple of years ago. It didn't mean there would be no new champions - there isn't going to be a constant "big 4" with new players taking the place of old ones.
The way I see it, it's a Big 1 (Djokovic) followed by a crew of 4 (Federer, Nadal, Murray, Wawrinka). I am loath to put Cilic in that chasing pack as he does not seem to be consistent enough to be a multi-slam winner like the others.