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.
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