That depends on whether you're looking for a completed ruleset or for the most up-to-date version. If it's the former, you can find PDFs for both 1st and 2nd editions on DriveThruRPG, but 4th edition is actively being updated with new books and adventures in the present. 3rd Edition, from what I understand of it, required a bunch of extras like cards and box-sets to play, which is why the license went from Fantasy Flight to Cubicle7.

Finally, the Zweihander RPG is sort of like the Pathfinder to 2e WFRP's D&D, extrapolating from its chassis and doing its own thing, including inspirations from similar dark fantasy settings like The Witcher, so it's not OFFICIALLY Warhammer unless you WANT to set your games in that setting.