And so I wrote a character-based 4D maze game, ala Nethack. Basically, it shows you a tetracubic slice of the maze using a 2D array of planes, suitably slanted to give an illusion of isometric projection. (If you like, this would be the view of a 5D being playing a maze game on a 4D screen.

http://eusebeia.dyndns.org/~hsteoh/4maze/4maze-1.txt
NOTE: the above URL has been updated.
In the screenshot, the player is represented by the '&' in the center. Walls are represented by '/', and parts of the maze hidden from view are represented by ':'. The '@' is the exit portal, and the '$'s are items that you must collect before you can enter the exit portal. And yes, there is visibility clipping, just to make things harder.

It's fun to be able to actually navigate a 4D maze without needing to understand projections and maintain your orientation, but focus on the actual geometry of the place. And I must say, this is hard. I thought I had 4D visualization down until I played my first non-trivial maze. The way passages can twist in 4D is just extremely difficult to keep track of, and it's extremely easy to get lost even though your orientation is fixed.
Anyway, if people are interested, I may put the code up somewhere. Currently, it needs a Unix-like OS (or CygWin) to work, unless somebody can be bothered to port it to conio.h or something like that.
Also, once I finish up with a new, looping maze generator, I'll add more gameplay elements to it, such as items (other than the gold you have to collect), and maybe some kind of storyline, and eventually a 4D city.
Comments?