Ditto. My solution so far has been to simply ignore the problem of modelling interesting objects--the only objects are components of the environment made of simple geometric shapes, and the game challenges are all related to navigation. I've been fiddling off-and-on with procedurally generating some 4D creatures for a sequel game (e.g., by using a genetic algorithm to evolve 4D shapes that can walk), but that's a long way off.
Yeah, I've mostly ignored it so far as well. I did get Dual Contouring working on 4D signed distance fields, but the resulting meshes are kind of janky. My thoughts are to eventually get boolean operations working on arbitrary tetrahedral meshes and do some CSG, or create a Blender-style 4D mesh editor.
Another game idea is The Incredible Machine in 4D, but it would be so hard to play, and even harder to design the puzzles.
