At this point we have both the GG KZSF postmortem showing 4.7 GB used (so we know at least 4.7 GB is available to games) along with Jonathan Blow's remarks about The Witness using "over 5 GB" of memory and that it "won't quite fit" on PS4 without streaming assets. Those are numbers inadvertently provided directly from developers with PS4 kits and active projects. GG providing the minimum of what can be expected for game use (6 cores and 4.7 GB) and now Blow's comments heavily hinting at the max (not quite over 5 GB). Speculative? Sure. But its speculation from what I would say to be good sources. Of course, its all subject to change before release.
Damnit, I let myself comment on this stupid topic again after saying I give up.
EDIT
Adding links and updating verbiage I quoted from Blow's Tweets:
GG Postmortem (Slides 6 and 15 in particular)
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