Are there higher dimensional analogues and generalization of space filling curves? Space filling surfaces (wrt 3D) or volumes (in 4D). Are they useful in any sense?
But Hilbert curves are complicated. If you have n dimensions, you can simply take the sequence of bits describing the position in each dimension and interleave them, forming a longer sequence of bits. It's not nearly as nice of a curve -- adjacent points along the line are not necessarily close in n-dimensional space the way they would be with a Hilbert curve -- but it's simple to calculate and proves that you can generalize this stuff to any number of dimensions.
This interleaving produces what is called a Z-curve or Morton curve. It's not continuous like the Hilbert curve, so mathematically you might not call it a space-filling curve but in CS-land, you probably would.