"My kindle 4 is one of my favorite possessions, I use it most days."
Word.
It's very rare that I don't fall asleep at night reading on my Kindle. I've read a TON of books that I never would have because I wouldn't have either sprung for the money for the physical version, plus I've found a metric buttload of neat books through the "Kindle book deals" e-mails I get. Fascinating stuff on military history, general history, murder mystery, etc., plus a bunch of classics that I've always wanted to read.
The only thing that might make me want to get a newer Kindle is if the newer ones handle graphics and illustrations better. Reading a lot of military history I come across a lot of maps, and my Kindle just doesn't handle those well at all. Very dim, can't enlarge them, etc. Basically it's pretty rare to get much of anything usable from the image. That's also true with black and white photos. Those don't render well at all.
I just finished a book discussing whether or not the German Blitzkrieg was actually a German military thing or a way for the Allies to explain how they got their asses handed to them early in the war (author makes a convincing case, but not a completely solid case that it was an Allied excuse/explanation).
Those kinds of books have tons of maps and photos, and they just don't do well.