I'm shocked noone has mentioned Manga Studio (or Clip Studio, which is the same thing)... I got it myself when I got my SP4, and it's probably the best art tool I know of, with all the extras of photoshop mixed in with some really nice inking tools. It does have a little bit of a learning curve but I've gotten the hang of most of it by the end of my first proper picture.
I always used sketchbook pro on my android tablet though, so I would also recommend that. It's a bit simpler, but a nice art program. I'd probably choose either of these over photoshop.