Reason for being non-free: severe restrictions on commercial/non-free distribution.
Current license says that version 4 will be released under a license that is "far more liberal in terms of reuse". Possible replacements: zrcube, Yafray, ayam (copyright trouble) +aqsis/pixie, partially Blender (discussion)
Version 4 will be a complete and total rewrite, and won't occur anytime soon.