Officially, there's a new form every degree all the way up. A 4th Dan is supposed to learn the form Pyongwon, a 5th Dan Sipjin, and so on. However, you have that many years to learn one form...it's mostly a formality. Higher ranks I believe are more about politics, experience, and contributions to the art than they are about learning.
I can't speak specifically for my old Master's curriculum, but I imagine his 4th Dan curriculum is like the rest of it. A bunch more stuff to memorize.
I'm noticing this in my Muay Thai/MMA class. There are some things that just translate really well. And some things that are completely foreign. And some things that I watched Muay Thai videos and tried to do the way they do them in Muay Thai, but turns out my coach wants us to do it in a more Karate/TKD style.
For example, leg kicks. In most of the videos and discussions, it's like swinging a bat. But my coach wants us to chamber the leg kicks. It's a different chamber than we've used, but I can feel the effect.
Then he has us do spinning kicks. I'm the best in the room at those. Even better than my coach, because spinning kicks are a luxury in MMA and a staple of TKD.
It's always nice after 45 minutes of looking and feeling like an idiot, when he says, "Ok, let's add a spin kick on the end of this combo." Now I can look like I know what I'm doing!