Why is there even a Big 4? Murray hasn't even won a Grand Slam and Djokovic is only saved by his recent success.. Nadal is playing very good now while Federer has 13 Slams - enough said.
Big 2 - followed by Djokovic, then by Del Potro/Murray/Simon.
Fed winning 13 GS is almost irrelevant. Sampras won 14 GS, so I guess Nadal, Fed and Sampras are the big 3?