Andy Murray has been undeniably successful at the Masters 1000 level, but losing each of the three Major finals he's contended is the usual reason given by those who'd exclude him from Big 4 status. But the record shows that he's head and shoulders ahead of the next ATP player behind him, and at least part of the reason Murray doesn't have a Major title to his name is that he's 1 of 5 in semi finals against Nadal, including 3 straight losses this year.
On the WTA side, I'd argue that the last four years have been Serena Williams and a supporting cast. The ATP has had headline rivalries: first Federer-Nadal, then in 2011 Djokovic-Nadal. None of Serena's opponents have been consistent enough to act as a true rival, as her sister Venus was at the start of the last decade, and Justine Henin was in the mid 2000s.
Like Andy Murray, Caroline Wozniacki is often criticized by reporters for failing to land a first GS title. At 21, Wozniacki is younger than Murray, who's now 24, and she broke into the top tier a year after Murray did (2009 vs 2008). Since her breakthrough at the 2009 US Open, Wozniacki has reached 3 SFs out of 8 - respectable, but not by any means near automatic.
If you're an ATP player, you know you'll see one of Djokovic, Nadal, Murray or Federer in your quarter of the draw when it comes out, and the odds are that you'll have to go through one of them to make the semis. If you're a WTA hopeful, seeing Serena Williams in your section of the draw will likely get your pulse rate up a couple of notches. If you have a laptop and a statistical bent, seeing another high ranking seed won't have you checking your favorite airline timetable for an early flight out.
