Amazon has quietly doubled the amount of onboard storage in new models of the Kindle Paperwhite second generation e-reader. Any new device ordered online from Amazon will have 4GB to store all of your books, instead of the standard 2GB.
There was a time when Amazon always included 4GB of storage in their Kindles, but this when they had features like on-board audio and audiobooks used to take up a ton of space. When the Paperwhite 1 and 2 were first released Amazon realized they could likely downgrade the amount of storage and nobody would really care, because all of your purchase were saved in the cloud. The only market to have a 4GB model of the Paperwhite 2 right out of the gates was Japan, and this was primarily due to manga and graphic novels taking up more space than your standard digital book.
Amazon has made no formal announcement yet about the subtle increase in storage. On the Kindle Paperwhite 2 product page they don’t even reference the exact amount of storage that is available anymore.