Ok, so I got my Kindle 4th gen today, but the seller gave me version that comes with ads. Fortunately, its really really easy to permanently disable them.
All you have to do is plug in your kindle to PC via usb and mount it. Afterwards open a terminal and change to the directory in which Kindle is mounted:
Than remove the .assets folder, which contains the ads related information.cd /media/Kindle/system
To pervent kindle from re-creating the .assets folder, make a blank file with same name in /system directory:rm -rf .assets
Your kindle will now be ads free forever. Infact, it comes with pretty background image when in idle.echo '' > .assets
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