Preliminary results for Havok 4 seem to be good. I’ve received no major reports of problems from anyone.
One thing I did notice in my own testing was that clearance in Havok 4 seems to be less forgiving. In Havok 1, if a car was slightly colliding with a wall, it would simply keep moving at a much slower speed. In Havok 4, it seems that the car stops dead in its tracks with even the slightest collision. So if your car is getting stuck all of a sudden you will need to increase the gaps between your car and all objects it passes by.
I have started a forum topic where I will post all Havok 4 issues I come across as well as any workarounds. Check it out here.