Ambient Occlusion crashes the Best renderer.
To test: Load up any model zoom in so it fills the document (so part of it disappears off the bottom), turn on Ambient Occlusion, then click Best Render (not BPR). This results in a crash every time.
It only seems to happen if the model fills the screen (ie, there is geometry on the last few lines of the render). However, it doesn’t crash if there is no geometry on the last few lines of the render.
Other than that ZBrush 4 is pretty solid on the Mac, but I thought I’d flag this bug up so others can check it out.