Super Smash Bros Melee had over 7 million copies sold on Gamecube which is exceptional when you know fact that Gamecube only had almost 22 million units sold in its lifetime.
I disagree that Uncharted 4 would outsell Super Smash Bros U... Uncharted 3 has 6.3 million copies sold on 80+ million PlayStation 3's so attach rate is below 10% while Super Smash Bros Melee had attach rate of over 30% on Gamecube.
Anyway... Wii U will easily surpass 8 million and 10 million is not out of the question...