After I thought I had it all sussed out I'm back to having issues with HDR.
As noted in here Jonathan Blow has said The Witness has issues, so
this post of mine seemed to have an answer.
However, now onto FF15...
1) PS4 resolution/RGB settings on auto, TV RGB on auto. Go to brightness settings on FF15 menu, both cactuars are easy to see on black and white square. Turn on HDR, white cactuar completely disappears...
2) PS4 resolution auto, RGB set to full, TV RGB set to full. Go to brightness settings on FF15 menu, both cactuars are easy to see on black and white square. Turn on HDR, both cactuars still visible, white one just a shade lighter. This possibly suggests a contrast/white blow out like what happened when I had my PS4 and TV set to RGB full and turned on HDR in UC4 when I first used my Pro and everything looked washed out.
3) PS4 resolution auto, RGB set to limited, TV RGB set to limited. Go to brightness settings on FF15 menu, both cactuars are easy to see on black and white square. Turn on HDR, white cactuar completely disappears...
Second set of settings shouldn't be compatible with HDR at all though, as at 4K the PS4 and TV cannot do RGB full with HDR??? What the fuck is FF15 playing at then? Why is the white detail completely blown out on auto/limited settings, but still visible when TV and PS4 are forced into RGB full?
Anyone with FF15 try it out? Use the cactuars in your brightness settings page like I have above. At this point I just think I'll turn off HDR full stop. From games seemingly implementing it differently, to PS4s and TVs having issues, the standard is a complete and utter joke. Half you folk out there with it on are probably either blowing out your blacks with crush, washing out your contrast/whites completely, or in the case of The Witness above ending up with blotchy purple mess on black scenes.
Set everything to auto and you think your PS4 and TV would handle things... Seemingly are they fuck with the amount of issues with HDR across games and TV brands. Honestly, at this stage I recommend everyone just turns it off.
edit: When thinking about it auto/auto for RGB settings essentially is limited/limited when a game turns on HDR. Hence how above when I force limited on TV and PS4 I have the white cactuar disappearing. So number 1 and number 3 above are essentially doing the same thing. So in my testing whenever limited RGB is used on the PS4 and TV, and HDR turned on in FF15, white detail is completely lost/blown out.
edit2: Some evidence another FF15 gamer cannot see the white cactuar when HDR is turned on
http://www.neogaf.com/forum/showpost.php?p=226121383&postcount=15281