Yes, yes, but they are also at the limits of the process, so any additions to the gpu design really have to have a justifiable benefit. There's no point in a huge set-up array if you can never skin your polygons in time...a set-up engine sitting idle is REALLY stupid.
5870 doesn't fit in this example, except to show how misbalancing gpu arrangement can lead to real big problems...and the 6870 and it's higher efficiency serves as the basis. The 5870 set-up engine wasn't even good enough to fill 5870 properly...much of the gpu is idle all the time, even in 3D.
But, why was it idle so much? Becuase only one-to-three SPs of the 5 in a grouping ever gets used.
This inefficiency is what precludes the switch to 4-D. But at the same time, scheduling for 5-D shaders is far different from 4D, with higher-order math capabilities...
How does the set-up engine affect 4D? Are you serious? What feeds the shaders? Fairy dust and troll hairs?