Hi all,
I've been busy writing a 4D first person shooter, and it does seem to work. There's not much to it so far, and only wireframe rendering, but it is playable. It uses colour to hint the extra axis. Once the target it is the right place you need to hold the mouse button down whilst you move the mouse left/right until it's the right colour (white). Space bar to fire. Links are:
Screenshot
Windows download
Mac download
If you have a go with it I'd be very interested to hear how you get on.