Hi there and welcome
The Kindle 4 is now replaced by the Kindle 5 which you can purchase from my link below
CLICK ME TO OPEN
Once you have it, when its started you register it with your current Amazon account and all your books are automatically loaded onto it.
They are linked to your account, not the Kindle so are safely stored online.
If you would like any additional information or assistance, please do not hesitate to let me know.
And if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.