It's not about Slam titles, it's about competitiveness.
For a long time those four players have been well above the rest of the pack. I mean, there are pretty much 4 'big' tournament types in tennis that (if fit) all the top players will be there and want to win - Slams, Masters, YECs and Olympics. If you visually represent how many of those each player has won in the last 4 years you get this:
15 - Nadal
14 - Djokovic
8 - Murray
6 - Federer
2 - Davydenko
1 - Soderling, Ferrer, Del Potro, Lubijic
Since 2009 Fed won 4 slams, 7 masters and 2 YEC, I don't know where you got that "6" number. I count 13
Nadal won 6 slams and 9 masters. 15, right.
Djokovic won 4 slams, 9 masters and 1 YEC, 14, right.
And Murray, in the other hand has, 1 slam, 1 Olympic gold and 6 masters, again, right 8.
Yes, Murray dominated with the other 3 players, nobody can't deny that, but we should concur Murray was way below these 3 players.
Since Federer (the one who won less of the 3) has 13 "big" titles, not 6, and Andy has 8, we find a difference of 5 titles.
It's a big difference, considering between Fed and Nadal there are 2 titles and they are the players who won the most.
But if we go to "bigger" titles, the number is really different 6 - 4 - 4 - 1.
I think ATM, yes, based on actual level, yes, Murray is one of the big 4, one of the players the oponnents fear the most, but still, he has a really long way if he wants to be remembered the same way as the other big 4. I think we can still talk about "big 3" players nowadays.
But of course, Murray can also be starting his best part of his career and win a lot of slams and "big titles".