In early December Sony announced that it had surpassed 2.1 milion units sold worldwide. Which at the time was a lot considering they only launched in a few regions.
Today, however, Andrew House announced that this number has doubled to over 4.2 million units sold. A staggering number considering this only accounts for November 15th through December 28th. It's also notable considering Microsoft announced their Xbox One sales numbers yesterday at just over 3 million.
Yes the Xbox One did launch a week after the PS4, so it will be interesting to see what the Xbox One numbers look like in the exact same time period.