mrzz, I hear what you are saying but think the reason it was a Big Four is that the gap between Andy and the rest of the field was at least as large as the Bigger Three and Andy.
Let's look at some actual numbers. The Big Four were the top four ranked players from 2008 to 2012. Here's the top five in each year, with ranking points. After the top five, there are two numbers - the gap between 3 and 4 and 4 and 5.
2008
1. Nadal 6675
2. Federer 5305
3. Djokovic 5295
4. Murray 3720
5. Davydenko 2715
1575/1005
2009
1. Federer 10550
2. Nadal 9205
3. Djokovic 8310
4. Murray 7030
5. Del Potro 6785
680/245
2010
1. Nadal 12450
2. Federer 9145
3. Djokovic 6240
4. Murray 5760
5. Soderling 5580
480/180
2011
1. Djokovic 13630
2. Nadal 9595
3. Federer 8170
4. Murray 7380
5. Ferrer 4925
790/2455
2012
1. Djokovic 12920
2. Federer 10265
3. Murray 8000
4. Nadal 6690
5. Ferrer 6505
1310/185
Andy was ranked 4, 6, 2, and 1 from 2013 - 2016, so aside from 2014 he has ranked in the top 4 in eight out of the last nine years.
Now if you look at the points, the gap between Andy and the rest of the field isn't that large from 2008-10, and is actually smaller than the gap between him and the Big 3. But one thing that also becomes clear is that Andy was consistently #4, while #5 was a rotating cast...this makes that gap a bit larger.
From 2010-12, the gap between Andy and the rest of the field is much larger than Andy and the Big 3, and in 2012 he's actually ranked #3, partially due to him winning a Slam, but also because Rafa was injured. But even so, the gap between Andy and #5 is substantial.
Maybe we can say that from 2008-10, Andy was "Best of the Rest" - and he was, quite consistently so. Again, the fact that he was consistently #4 while #5 rotated speaks a lot to his ability.
From 2011-12, he was "Worst of the Best," which sounds like a downgrade but is actually better. Roger struggled in 2013 but righted himself and despite being ranked below Andy in 2015, was the better player. Rafa surged in 2013, so Andy was clearly behind him and Novak, and then behind Novak and Roger in 2014 and 2015 (despite the ranking), so he remained "Worst of the Best" form 2013-15. And of course we have to factor Stan into the mix, especially starting in 2014. Let us not forget that Stan has won more Slams from 2014-16 (3) than anyone other than Novak (6).
Novak stumbled in 2016 and Andy took advantage. I don't think we can say he was the leader of the pack, so Big 2 works for me. Clearly Rafa is diminished, and Roger was injured for most of the year and there is no telling how he'll play when he returns.
The point being, going into 2017, Big 2 works just fine. It was true in 2016, and looks to be true to start 2017. Perhaps Roger and Rafa resurge and all of a sudden we have a Big 4 again, at least for awhile. Or perhaps Novak finds his 2015 form again and/or Andy reverts to his erratic form. Or perhaps some other player rises up and challenges. But right now, it is a Big 2, and clearly for the last nine years, it has been a Big Four - with Andy belonging more in the elite than outside of it.