It's not exactly a fair comparison, but the four million unit mark is still a pretty good place to be.
The Nintendo 3DS had a bit of a botched launch. It was priced far too high for its own good and was released at a kind of a strange time - spring releases are just weird. Despite all this, the system has been doing well.
The 3DS has hit a total of four million units sold in the United States. It's an impressive turnaround for the system, and it's a number that puts the 3DS ahead of the Wii in terms of first year sales. That's a lower number than Nintendo expected, but one that they're sure glad to get to.
The Wii comparison seems impressive, but it's important to recall just how unavailable the systems work back when they launched. Stores couldn't keep them on shelves, true, but that's because Nintendo didn't make nearly enough. The reason the Wii sold so little in the first year is that there were no more left to sell.