That is the start/restart screen that you see only when the Kindle has been reset or restarted. It will also display if the Kindle updates to newer firmware. If it got stuck on that screen then I would guess there is a firmware issue, perhaps some files became corrupted. Did you try to restart it again by holding in the power button for approximately 20 seconds? If not then give that a try.

Do you know which firmware the K4 was running before the freeze? Was your K4 jailbroken?