According to TechCrunch, the Kindle 2 had already sold over 300,000 units just two months after it went on the market in February 2009. That's an average of over $100 million in revenue from the $359 Kindle 2s. The Kindle DX is selling for $489, so you can imagine what kind of profit the DX will bring in if it does well.
The nearly $4 billion estimate is all speculation, and anything could happen in the next three years. Amazon could change the pricing on the Kindle, or Kindle alternatives could become more popular -- leaving Amazon in the dust. Only time will tell if this estimate is correct, but at the rate things are going, things are looking good for Amazon.