After having played 4th Edition extensively, I'm afraid I have found that nearly none of the rules additions made in 4th edition work.
At all. Some of them are so ineptly imbalanced it is a disgrace Cubicle 7 allowed the game to release in such a state.
Just to pick the largest elephant of many in the room: Advantage.
It. Just. Doesn't. Work.
Advantage in WFRP4 is a mechanism whereby you keep getting +10% bonuses (the game uses percentile dice) each time you succeed. When you lose you lose all of the advantage you've accumulated. This massively transforms the game, and not in a good way. In fact game-play becomes unrecognizable from the rather down-to-earth game WFRP has always been, and WFRP4 still pretends to be (even though it secretly wants to be more like Warhammer Fantasy Battle).
If your players are even a little trying to act in rational and effective ways, they will immediately start chasing Advantage, rather than performing actions that make sense in the game and in the scene. You will always go for the easy shot, since you can always attempt more difficult actions later, when you've amassed a game-changing +40% or some such... You will also start looking to prevent the monsters from going on an "Advantage avalanche", meaning you will find yourself taking actions from a pure meta perspective. ("I know it makes no sense to whack this Orc here, but trust me, he would have become unstoppable if I didn't").
That kind of meta game might have worked for the Hulk and Black Widow - but it falls completely flat in a game featuring ratcatchers and guttersnipes.
Advantage is just one out of far too many rules additions that have been added with seemingly no play-testing, and no thought on how all these new rules add together. 4E also introduces opposed checks. A given bonus (say +20%) that works fine in 2E is almost always too large in 4E. This seems to have flewn right past the devs - most numbers remain in the same ball-park (if not identical). While getting the above +40% in 2E would be good - great even - it would still not save you against the monster, since you don't oppose its rolls, and you have limited chances at active defense. In 4E, it changes everything, and basically lets you go on a super-hero spree that I can't believe anyone want.
WFRP4 is a horrible mess of a system. Nearly every facet of the 2E rules have been made more complicated, with loads and loads AND LOADS of little niggly special conditions and exceptions... but Warhammer doesn't need a massively complex game. (Don't get me started on how fiendishly complicated resolving an attack can become once the PC and the NPC becomes moderately complex - there are Talents who only activate on a success, separate from a win; there are modifiers from talents, equipment, injury, circumstances, the enemy... it all too easily becomes nightmarishly complex. I'm talking D&D 4th edition levels of cluttery complexity)
Weapons, Armor, Critical Hits, Monsters, Talents... every one of these areas has been cluttered down with loads of little special conditions and modifiers that make it much much harder to gamesmaster WFRP4 than WFRP2.
Wizards start out cripplingly ineffective, only to become entirely unbalanced powerhouses given experience and a minmaxed talent selection. Warhammer has always featured a boring spell selection far too combat-focused, but in WFRP4 you don't even need the spells with higher Casting Numbers. Dart and Blast will remain far superior no matter how powerful you become, because - again - several rules changes were implemented at the same time without anyone considering the complete picture.
Warhammer doesn't need most of the rules additions offered by WFRP4, even if they had worked.