Yup, as I said, I think it's a trick question. Or, well, not really a trick question per se, but ... the underlying symmetry geometry of the 4D spacetime manifold is not directly perceivable or navigable from 3D space, such that from the viewpoint of the observer you're always somewhere in the middle -- you cannot get to an edge. The edges, faces and vertices are always, effectively, about 20 billion LY from you, no matter where in the 3D universe you actually are.
"Which point in the 3D universe is in the center of the 4D universe?" "All of them, and none."
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"Which point in the 3D universe is in the center of the 4D universe?" "All of them, and none."