Bought one to write
this review but for my day-to-day reading I'm going to stick with K3 (Keyboard). In everything except for size and weight K4 is a downgrade compared to K3 and even K2 in many aspects.
Unless you absolutely prefer size to keyboard or absolutely don't want to spend more than $79 on eReader or you absolutely don't use 3G, I'd suggest waiting for Kindle Touch since this is the one that really seems like an upgrade.
For me personally the biggest turn off is complete lack of 3G.
Apps will catch up eventually since the blocker there is keyboard-centric UI (that can be changed in many cases) rather than memory limitation