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are considered fully compatible with 4th edition.  I assume it's only those with the new 40K lettering on them, but what about the new IG one which came out just before I disappeared into the warp? Witch hunters, CSM, etc.

Three years in the warp is a long time and I don't want to have to throw away any more books than I have to.

As far as I know, they are all fine. If you look on the errata section on the workshop website there are FAQ's  which explain how to use the old codices with the new rules. Guard,  WH are all pretty muich fine. The ones with the biggest changes are blood angels etc. but you shouldn't have to throw anything away.

J

They're all fully compatible, assuming you mean work legally in 4th.  Several of the the 3rd codices have been replaced (Space Marines, Nids, Tau, and very soon Eldar), Dark Angles and Dark Eldar had "2nd edition" codices that dramatically improved their playabilty, and the CSM codex has had a few "ghost" revisions.  ghost revisions are changing text with rules consequences with the new printings, but never really telling anyone.   Most notably, the obliterators have daemonic toughness, not true T5.


Now, if by compatible you mean do they work competitivly in 4th edition, you get a different story.  Orks are notoriously underpowered, Eldar have only a few builds that compete, etc.  You can build a powerful army out of any codex, but you're options will be quite limited while you do so.

Every codex that has Warhammer 40,000 and then the army nameat the top, and then the word codex in the corner is 4th edition.

until replaced with a 4th ed codex print, all 3 rd ed codices are legal.

The ones replaced so far:
SM
Tau
Tyranids
Eldar (to be released the beginning of november)

check out the faqs for compatibilty purposes (such as deathwing and whatnot)
http://us.games-workshop.com/games/40k/gaming/FAQ/default.htm