My thought process says that this 3.5 is not ONLY for the Operating System but for other functions as well. For example the 15 Minutes of continuous recording of video and keeping performance up for that. That is one thing, I am sure there is more.
That is just a part of the OS, also MS has 3 separate OS's which take up 3GB and Sony has one which takes up 3.5? (Xbox also has continuous record)
People seem to be talking about Marketing a lot and realistically neither have actually started their big marketing push.
They have made some PR stunts like PS4 taunting Xbox One about used games and Xbox One hyping up the cloud, its small trinkets at the moment.
They are releasing little bits of information to keep people interested and talking about it, Sony and Microsoft both love the console wars.
Like Apple, they 'accidentally' leave a new iphone at a bar somewhere and its details are 'leaked' then it slowly releases small bits of confirmed information here and there.
It keeps people talking about it and relevant, if they told us everything about the console 2-3 months ago we wouldn't be talking about them now.
Also the 4.5GB free RAM is a hit on the PS4's hardware dominance if it turns out to be true, its doesn't mean the Xbox One is now better but the PS4 did drop a little bit.
The original suggestions were the OS was going to be 1gb, I thought this was actually a bit much because the PS3 OS was tiny and it was fine. I don't think the PS4 would use much either I think its to do with the quick-switching.
Specifically when quick-switching between gaming and blu-ray movie (high-res video) because they want to make everything instantaneous you could play a game then switch screen back into the OS, or a blu-ray movie.
So all three would need to run as background processes or stored in RAM for quick access. So maybe 1gb for OS, 1.5gb for OS apps, 1gb for blu-ray and 4.5gb for games?
I would much rather have that extra gb available for games rather than reserved for instantaneous blu-ray if that is the case.