Page 232 of the user manual "No sound is output when using ZONE: Digital signal audio input from the HDMI connector or digital audio connectors (OPTICAL/COAXIAL) cannot be played back. Use analog connections"
The receiver has one DAC. Its allocated to the main zone so you can do Blu-ray and other digital source playback. Zone 2 doesn't have its own DAC. There are a couple of reasons for this. First is cost. The sort of DACs that can handle all the source signal types that an AV Receiver might see are not cheap. Second, Zone 2 out is stereo only, so if the DAC decodes a multi-channel signal the amp then has to process it down to stereo as well. Again that adds cost and complexity.
There are (or were) some top-end receivers that did feature dual DACs, but they were all in the £1000+ category.
For something like the PS4, my understanding is that multichannel goes to whichever of the HDMI or Optical outputs you've selected as the primary. In your case that would be the HDMI connected to the receiver. The other output (Optical now) can output stereo. Clearly, Optical is stil a digital connection, so you need one of the little Optical to Stereo Audio DACs for £5-£10 to convert optical in to analogue stereo. At that point you then have an analogue signal to run Z2 for PS4.
You can do the same sort of trick with a Sky Q main box. The minis have an analogue output on 3.5mm jack. Sky+HD boxes vary with design. The older HD boxes have analogue stereo out on phonos, but the later boxes just had Optical/HDMI/SCART. You can use SCART though, either with a SCART to composite + stereo audio cable or with a plug-in SCART adapter that gives phono sockets for the same signals. I don't know if VM boxes can do the Optical/HDMI thing. There's inexpensive ways around for most gear with the exception of devices that only have a single HDMI out, an nVidia Shield for example.
