I've actually learned a lot from 4k code. It can be hard to get a handle on, but can be very rewarding.
Of course you need to realize that for it to be viable, 3d in pure java needs to be very low-res (in every sense).
Making old-style (as in very old) 3d games could be viable and interesting, but anything more complex will probably hit a brick wall soon enough.
I personally have looked at the DieZ code myself, and think such a 3d engine can be interesting for certain retro-style game ideas I have. Love the grass effect in particular... But no, I can't explain much about how exactly it is doing what it is doing.