The Pro Q3 was already top-tier, but this steps it up even further.
With great new features like spectral mode (essentially turns it into a Soothe 2, or like the dynamic function but more fine tuned), a fantastic new view mode where you can see and edit all instances of the Q4 in one window, and a great new saturation feature (subtle and warm) which makes it sound more like a Pultec EQ rather than a clean, digital EQ.
All in all, well worth spending the extra to upgrade!