If you want to honestly enter some sort of "crow-eating competition" as to whether or not there's a "significant" graphical difference in multiplats between PS4 & the 'Bone, you're gonna have to make some sort of outline. Otherwise, you won't even be able to argue "moving goalposts" come November since they won't have even been planted to begin with.
I'd go with something straightforward:
-If both versions look absolutely identical from a graphical standpoint (i.e. a screenshot of the exact same moment in both versions look entirely indistinguishable from one-another), then the one with the highest average framerate in some sort of demo wins (say, an in-engine cutscene for example)
-If there's resolution differences, AA differences, or differences in other IQ variables, whichever one has the higher values wins. [You might have to establish some pecking order for AA preference while you're at it, lol]
-If there's differences beyond that, as in quality of assets or visual effects, the whole discussion gets a lot more specific and nuanced for that scenario, and really starts to come down to preferences, where even a seemingly ridiculous assertion that "x looks better than y" becomes defensible on the grounds of pure subjectivity. (e.g. "I think PS1 games look better than PS2 games because fuck you, it's my opinion")
Just throwing mental vomit out there for whatever it's worth.