OS4 for G5 exist

Assuming you mean that A1X1K thingie .... much closer to a modest G4 than any G5 in terms of performance.
MorphOS-team at one point decided that it's more important to support decent laptops (PowerBooks) than hunting down more power only sensible for apps we don't have.
Sure I can run AROS from 20-core-AMD-you-don't-need-no-heating-anymore down to some obscure ARM-SOC (but I have to be ultracarefull to choose the right HW when I don't want to go hosted), but what than

Run those same old 3D-demos at 400fps ?? Or maybe that minimalistic OWB-version (as long as Wanderer doesn't crash on me) ?
All 3 flavours allready have enough HW to run on, whats missing is the SW to make the systems really useable and in the field I see MorphOS clearly ahead (to some great extent due to Fab's OWB).