Given a slowly expanding four-dimensional universe, where the bulk of it is filled with solid matter, and nexi and streams of energy interpenetrate the bulk and carve out hyperspherical and hypertubular voids that may be filled with liquids, gases and vacuum, what would the geography of this world/universe be like?
In particular, how does having four spatial dimensions change the nature of this world's geography in comparison to three-spatial-dimensional worlds? Are there any geographical features that occur only in 3D space, or only in 4D space?
How would the dimensional stresses of the expanding universe be relieved in the solid bulk, and what effects would that have on the universe's geography?
This is part of the related series of questions, What would the periodic table of a 4-Dimensional universe look like? and What would organisms be like in four physical dimensions?.