A STEMS exporter for burning songs for use on Traktor or other systems which use the STEMS file format. The exporter could based on track groups, send tracks or some other grouping.
http://www.native-instruments.com/en/specials/stems/
http://djtechtools.com/2015/03/30/stems-a-new-multi-channel-audio-format-for-djing/
Exporting stems is already possible in Renoise. It is up to you to limit your song to 4 stems only 
How would you do that? With the ‘render each track into a seperate file’ option?
Is there any way to bounce down tracks onto a standard track and not a send track?
If you say wanted 8 tracks bounced down to 4 stems.
I would probably just group them and then use the group stems.
grouping all tracks into 4different groups is right thing to do, also you can do final twist on mastering using stem creator tool