In my opinion the KS4 wins over the Nova II. The KS4 sounds a little brighter, less glassy and has more clarity. Id also say that it sounds subjectively warmer...in a VA kinda way. I used a KS5 at one time....
Now, If your wanting to know how it sounds...then I strongly recommend you get this >>
http://www.howardjones.com/adverts/dvd/dvd.html Its a great dvd....spanning 3 1/2 hours. There are 4 sets in total (acoustic, retro, electro and band). In the electro section you see him playing 4 Novation KS4's and KS5's.....and it gives you a real idea to how they sound.
DS