I own 3rd, and I purchased the books for 4th ed on ebay. I read the both rulebooks side-by-side, line-by-line, and they are identical. The only things you need to convert 3rd to 4th are the mission book, and the new punch card with boarding torpedoes, new room, and breach counters.
I also added to my set a dice bag that I painted with puffy paint in the shape of a blood angels winged blood drop. I use it for the command chips, to keep them more secret. Just drop 'em in, shake it up gently, and reach in blind.