It is clear from the FAQ that official versions for 4GB do not run Kindlets, but the idea is to try to install recovery image from 2GB version on 4GB version - if this succeeds, then probably Kindlets will run on 4GB version.
KOreader is a very configurable reader, but in a number of important areas the native reader is orders of magnitudes better. For example, KOreader is quite slow opening a book (can take a couple of minutes), and many times it just either freezes or crashes the device. And it simply cannot handle really big books (try to open this one:
https://tools.ietf.org/ebook/rfc.mobi). The native reader doesn't even pause during those operations. So unless there is no choice, I prefer the native reader.