[Anaraxes]

(1) is setting-dependent, and could be any number you like. Enumerating all the ones mentioned in published fantasy would produce quite a long list, even if we count "infinite realities" as just one per author. Trimming the near-duplicates and close analogs would be its own chore.

You don't need Elemental Planes to support elemental magic. I have no trouble imagining "fire mages" that are adept with fire spells without there being any need for an entire plane of fire, or intelligent personifications of those elements.

(2) Are we excluding 3e, so the question is just "planes mentioned in GURPS settings books published specifically for 4e"?

If "hyperspace" counts, do we separately count "jumpspace" (as for GURPS Traveller ISW)? (I'm using the definition of "hyperspace" as one in which ships can willful maneuver and often interact with each other, like Star Trek "warp" and "subspace", whereas "jumpspace" is a state that's pretty much locked in once you jump.)

Should the planes be "real", in the GM's view of the setting? Roma Arcana, for instance, will entail belief in a number of planes of afterlife according to its inhabitants, but they're not necessarily "real" as far as the GM is concerned -- even if spirit magic like speaking with the dead works, that doesn't mean that any particular view of the afterlife is correct.