I know this term is common usage now and I am not critizing the opener of this thread in any way, but frankly I think it's still rather early to talk of "big 4". Let's have win Murray at least one slam before calling him "big".
Interesting that you say this because I agree with most. There are several reasons why Murray isn't part of a "Big 4" in my opinion.
1. Murray has no Grand Slam title.
2. Murray hasn't beaten 2 or more of the other 3 in one tournament while the others have done that.
3. He's ranked (in points) closer to the number 5 than the number 3.
Also, he's only been hitting form since/after Wimbledon and has yet to prove himself consistent over a longer period of time (the same thing holds for Djokovic, but to a lesser extent).
On the other hand, if Murray wins the Australian Open all it is likely that all my three reasons listed above hold no value anymore.
This is all slightly off topic though.
EDIT: Oh and of course, he doesn't have a high sanyaku rank