I've played WFRP 1st, 2nd and a little bit 3rd editions before this one and 4th is by far the best, the most complete, the most lore coherent, the most sophisticated (with optional levels of complexity) and the most fun of them all.
I like DnD much, especially 5th edition, but WFRP 4th is just another league. Previous editions suffered in my opinion to a little bit boring combat. Deadly, yes, but still not that fun enough, especially with multiple miss-miss rounds. Not anymore. Now it's just as deadly as always but apart of that super fun and with plenty of choices to make.
Another cool thing is whole career status, earning and endevours mechanics along with career advancements that don't enforce you to change careers.
Theoretically unlimited leveling (no skill or characteristic cap) is another cool thing too. I probably wouldn't need it that much often, but it's cool the system can handle that.
This edition is just perfect and a must have for any RPG enthusiast.