I wouldn't be at all surprised if they built a 4D. PKI is all about building unique rides. Remember The Bat? Delirium? FOF? Screamin' Demon? TR:TR? All were one-of-a-kind when first built. Well, FOF was two-of-a-kind, but you know what I mean.
I have a problem seeing them build a B&M or Intamin hyper. Something so... standard... just wouldn't fit. Besides, they already have a hyper.
But I digress. Whatever their next coaster is, it'll be good.