ADAT Optical 8 CH Out - 8 CH In... ends up 'stereo' In..?

Neverending story… :wink:
On the slave Cubase VST connections, create 4 stereo outputs, connect them to 4 Stereo ADAT pairs. Send esch VSTi to the desired output. In your slave card´s mixer make sure every playback output foes to the corresponding Hardware ADAT cgannel(s). Just as simple as with discrete analogue outputs.