It took me a whole night and then I finally realized what the heck is going wrong with the pressure sensitve stylus support in Toon Boom Studio 4. It seems like that Toom Boom Studio 4 does NOT like the multiple monitor setting, and whenever I have multiple monitor setup, the pressure senstive stop work. Switch back to single monitor mode resolves the problem.
Great, now I can really get rid of Toon Boom 3, since the gradient color fill doesn't work when there's more than 2 colors.
But I am still struggle with how the peg think works with toon boom studio, and how the whole camera thing works. Maybe I just need more time to soak myself into it.