Also, the power systems from 3rd Supers and Psionics have changed completely, but both still have cool ideas. The details about how combat works have changed particularly notable are changes to how active defenses are calculated when used in conjunction with armor and advanced combat options, making 3rd Martial Arts a challenge to utilize. Some books will be harder to use than others. Another example, 3rd GURPS Magic presented some magic ideas and systems that still haven't been replicated in 4th. For example, maps and physical write ups for locations and such are useable as is (the game's scale for size and distance didn't change between editions). But even mechanically, you could still get some mileage out of certain books. The books are gold for background, info, and flavor text, as all of that would still be usable, of course. I would say that, more or less, you can get some use out of any 3rd book, with some exceptions. Honestly, GURPS supplements are as useful for their non-mechanical information as their game rules. You'll likely have to houserule a good number of things, but, given that the core mechanic of GURPS (roll 3d6, lower is better) remains consistent through both editions, the game can tolerate a surprising amount of mixing. Hell, you could even have NPCs (or PCs, if you want), creatures, powers, whatever still use 3rd rules and mix with 4th rules. With almost anything else, a rough carryover will work.
STATS FOR ZOMBIES IN GURPS 3RD EDITION PDF
The only time you would need the level of detail provided in the pdf is if you're translating a player character(s) over from 3rd to 4th and want to make absolutely sure that the points all balance. In my experience, you can mostly get away with eyeballing or approximating a lot of things in the translation between editions. Like a lot of things in GURPS, it looks intimidating at first, but don't be thrown by it.
STATS FOR ZOMBIES IN GURPS 3RD EDITION FULL
Here is a free official PDF with the full list of changes from GURPS 3rd to 4th.