Thanks TheWarrior and PENG for your replies!
PENG, I just went into PS4 settings and yes, PCM was the audio output, so I switched it to DTS. I then popped the GotG blu-ray back in and yes, I was able to select Dolby Listening Modes and Neo:6.
I then went a bit further as TheWarrior had mentioned and bumped the crossover frequencies to 80Hz. And, what? Now as i cycle back through Listening Modes, these are my options:
Multich, Mono Movie, TV Logic, All Ch Stereo, Full Mono, Theater-Dimensional, Mono.
I had also adjusted the individual speaker volumes (Setup->Speaker Adjust->Level Calibration)
Any idea why it took away the Dolby options?
For reference the Speaker Config menu shows:
5 options (Front, Center, Surround, Surr Back, and LPF of LFE) all set at 120Hz (or 80, but it didn't seem to change anything for the better.)
I just turned off my PS4, and the red Dolby logo is back displaying on the receiver (blue screen, no source) I can choose Dolby PLIIx and DTS Neo:6. Turned it back on, showing the Dolby logo still, but as soon as the movie starts playing it's gone.