I just upgraded from Manga Studio Debut 3 to Debut 4, and let me tell you that it is a sexy, sexy thing. They took everything I liked about the older version and made it better, more responsive, and easier to use. (Auuugh I sound like a press release.) I always used Debut 3 to do my inking for personal projects and for work--it was fun, but still a little difficult at times. Now I am literally running around looking for excuses to ink things in the new one. It's that much fun.
Best of all, if you have version 3, you can upgrade to 4 for only twenty bucks. Linky go here: [link]
I know the program is a little counter intuitive, so if you do happen to get it and run into trouble, hit me up with a note and I'll see if I can sort you out.
I. Love. This. Program.