I've only got about a dozen books on my K4, so that shouldn't have been the issue. The book that froze is a typical sized, unmodified AZW straight from the Kindle bookstore. Very few images and nothing unusual about it. My issue was probably just a glitch.

I will say that occasionally my WiFi signal strength will drop to nothing and that can cause my Android phone and iPad to act sluggish and/or freeze, but it has never adversely affected either of my Kindles. Of course my Kindles rarely use the WiFi even though I keep it on on both of them.