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
i.e. the big 4 have won 43 of the 49 blue ribbon events in tennis in that period. Murray has won 8 of those. Yeah there is only one Slam in there, but it is still more than double what the rest of the tour has been able to take off the top 3. That means he is the only one consistently able to play with the big boys. He is further above the rest of the tour than the other 3 are above him.
So yeah, although he is the runt of the group, Muzz is definitely part of the big four dominating the tour. Has been for a long time. The rest of the tour are playing for scraps.
You got it better than everyone else, the key word being competitiveness.