I'm sure you know that the K4 is the stripped down model. It doesn't have the touchscreen of the Kindle Touch or the keyboard of the K3. It also lacks speakers, and that means it cannot play MP3 or use Audible audiobooks.
But there's one hardware deficiency that's not obvious until after you crack open the K4 case;
the K4 has less RAM than the K3. That means that there are games and apps that you simply won't be able to run on it. BTW, you also might have trouble using apps that require the keyboard.
Update: It turns out that my source was wrong on the RAM. the K4 has the same amount as on the K3. But the app limitations are very real.
I was tipped to this earlier today on Twitter, and just to satisfy my own curiosity I decided to send all my apps to my K4 and see which ones would run. It turns out that Amazon are a step ahead of me.
I only own 25 apps for the Kindle (that's really not so many considering that there are now). Amazon won't even allow me to send half the apps to the K4. Clearly they've already checked for compatibility and the ones that won't work on the K4 are simply blocked.
I was going to put together a list of compatible apps, but I'm not sure that it's worth it. Amazon already checked for compatibility. If you want to know if you can use the app on your K4 then just check the listing on Amazon.
Some won't even need to be checked becuase they obviously require too much RAM (or a keyboard). Take Monopoly, for example. This is an app that clearly needs more RAM than the K4 can provide. And then there are others like NotePad. This app requires a keyboard so Amazon won't even let you try to download it to the K4.