This Article – http://nypost.com/2016/08/29/this-us-open-may-be-the-last-gasp-of-mens-tennis-big-four/ made me think a little.
From 2004-2016, the Big 4 has won 3 or more majors every year except for 2014 (Cilic and Wawrinka broke through). In 2016, all 3 majors to this point have been won by members of the Big 4. Novak Djokovic has won 4 Masters 1000 events, and Andy Murray has added a gold medal to this haul. The gold medal in 2008, 2012, and 2016 was won by a member of the big four with a bronze in 2008 and silver in 2012 going to other big 4 members. Sure Federer didn’t win a medal in 2004, but there was no big 4 in 2004. Even with no big 4, Federer won 3 of 4 majors in 2004.
Of course Federer is older and injuries seem to be impacting his career for the first time. Rafael Nadal has missed big tournaments due to injuries since 2006. Novak Djokovic may have some injury concerns entering this US Open, but he did win Toronto. The dominance of the Big 4 will be coming to an end at some point, but to peg that decline to 2016 when Djokovic and Murray have more or less won everything of note except for Monte Carlo (Nadal took that!) and Cincinnati (Cilic) seems premature at best.