Amazon has not been very clear about warranty rights after the software on a Kindle is modified. A customer support official declared:
"Customer Service does not support or provide instructions for adding custom images to your Kindle to be used as screen savers. However, should you choose to do this on your own it will not void your warranty.". This was referred to Kindle 2 - now that the 3 and 4 ship with ads, it might be different. In particular, the Kindle Terms of Agreement states:
"No Reverse Engineering, Decompilation, Disassembly, or Circumvention. You may not (i) modify, reverse engineer, or disassemble the Kindle or the Software whether in whole or in part, (ii) decompile the Software in whole or in part (except to the extent such right cannot be excluded or limited by law and then only when the express permission of Amazon has been sought and refused), or (iii) create any derivative works from or of the Software, or bypass, modify, defeat, or tamper with or circumvent any of the functions or protections of the Kindle or Software or any mechanisms operatively linked to the Software, for example, by augmenting or substituting any digital rights management functionality of the Kindle or Software."
In a word, it's probably true that you'll void the warranty. Anyway, given how fast new devices are being released, I'd change the screensavers and enjoy your Kindle. If something goes wrong, you can get the K4 at some point.
Disclaimer: I might be biased in my advice, because I created a website about
Kindle Screensavers. There is also an application to
automatically change Kindle Screensaver