It really depends upon how granular you want to be, because I think it is meaningful to look at different degrees of dominance. For instance, Novak's huge dominance in 2015 vs. his lesser dominance in 2012, when Roger, Andy, and Rafa were closer to him.
Anyhow, there's always a sliding scale depending upon what time frame you're looking at, which sometimes overlap. For example, we can say that Roger was the overall most dominant player from 2004-09, Rafa from 2008-13, and Novak from 2011-16, even though Rafa was more dominant than Roger in 2008, Roger than Rafa in 2009 and Novak than Rafa in 2011, Rafa than Novak in 2013 and Andy than Novak in 2016.