Not really.
I am Elemental Shaman and Elemental Shaman ONLY, I have like 5 characters, but they are Elemental Shaman.
While I am not saying everyone would play one class only, but there are likely to be people that NOT wanting to try DH for whatever reason (maybe prefer range only?), if we include a % of these people, let's say 1/4 of the players do not want to play DH because they are range only or tank/healer only.
So the 3.9mil players are only the 75% that would try out DH, so total sub would be more at 5.2mil.
Blizzard no longer releases subscriber numbers, and making up assumptions based on a single class like this is hardly indicative of proof of anything. There isn't anywhere useful such a thread can go.
If Blizzard chooses to release numbers again, threads can be opened related to the official, stated numbers.