Mario Kart 8 is at 7.50 million and still selling, it should be able to surpass 8 million when all is said and done.
Uncharted 2 & 3 both sold over 6 million but under 7 million and Uncharted 4 will need to sell at least 8 million so that would be a pretty decent sized increase from previous installments but still feasible.
I'm torn and it should be a pretty close battle, I think there will be less than a 500,000 gap between the two.
When the herd loses its way, the shepard must kill the bull that leads them astray.