Actually VGC is overtracking Skyward Sword. It was at like 3.7 million as of March 2015.
http://www.neogaf.com/forum/showthread.php?t=963700
Skyward Sword was super frontloaded. It really didn't do much after those first couple of months and that 3.5 million was shipped so that shipment lasted for a few more months.
So it seems to me like 4 million is a good objective for Zelda Wii U.







