Scott Kim, the author of Inversions, wrote a program about 4 or 5 years ago which ran on Tenex, and allowed one to rotate 4-D skeletal objects and view the 2-D projections (on an Imlac display). I don't know what language it was written in. It was apparently quite good at allowing one to achieve an intuition for 4-D space. I'll send him a message and ask if his program is available anywhere for playing with.

--Teri Pettit at Xerox OSD