Here's the deal...somewhat. From a dev at one of the biggest in the industry. Just a messenger here.
Dev still a little cagey to give out 100% concrete numbers unfortunately like I was previously told, so I apologize ahead of time that they're not as detailed, but these are supposed to be the closest to detailed info I get on the OS footprint:
-At most, the OS uses 1.5GB, with a good amount of that helping the HD recording feature and on the fly switching/ multi tasking
-4.5GB is indeed about what's available for devs at present.
-1GB is set aside for futureproofing OS operations down the line.
-1GB for developer reserve, should they need it
And finally as an important point, I'm being told the faster RAM in the PS4 does in fact make a huge difference.