As Fieari already alluded to, it's not quite that easy.
Actually, if you want a 4D "die", you don't want just any extrusion into 4D. What you really want is a 4D equivalent of the
platonic solids, aka regular polyhedra. So what you're after are the
regular polychora. Cool name, huh?
A polychoron is a 4D polytope, and a polytope is just a generalisation of the sequence point - line segment - polygon - polyhedron - polychoron. The "surface" of each regular polychora consist of regular polyhedra. Luckily, you can find more info on these beasties on
Mathworld. Apparently, there are only 16 regular polychora (just like there's only 5 platonic solids: d4, d6, d8, d12, d20).
For example, the hypercube is constructed by joining all the vertices of a cube with the corresponding vertices of a second cube offset in a 4th dimension. This polychoron is bounded by 8 cubes: the two on each end, and 6 more formed by connecting each side of the first cube to the second cube.
The other easy regular polychoron is the pentatope: take a tetrahedron (d4), and add one extra vertex equidistant from the other four nodes in 4D. It's pretty easy to see that the pentatope consists of 5 vertices, 10 edges, 10 triangles, and 5 tetrahedra...
Now... of course it all depends on how you want to roll these 4D dice! I would recommend a 4D table top, which means that the dice will come to rest on one of their polyhedra, rather than one of their polygons. Besides, a 4D table top also provides more space for sodas, charactersheets, etc.
