Hundreds of thousands, let's say 500,000.
A poster suggested some groups have only 1 or 2 PHBs, so let's say all groups buy only 1 PHB. Let's say all groups have the good number of 6 people. (1 dm, 5 players)
That's 3 million 4e players, out of 6 million D&D players, with very very optimistic...