Though it cannot be
visualized (at least not without several accompanying
headaches), the
hypersphere can be dealt with
mathematically.
A couple of mathematical properties of the hypersphere:
It has a hypervolume (measured in distance^4) of π2r4/2, and a surface volume of 2π2r3.
A solid angle of a hypersphere would be measured in hypersteradians, of which the hypersphere would contain 2π2 in total. (It would seem that 2π radians in a circle and 4π steradians in a sphere were setting up a nice simple pattern and that 8π hypersteradians would be next, but actually the N-volume, N-area, and number of N-radians of an N-sphere are all related to the Gamma function and the way it can cancel out powers of π halfway between integers.)