Like I stated earlier its not that easy.. The OS itself probably doesn't use more than 1GB of that memory, most of the memory reserved is for external Apps. The further down the road they get the harder it is to take away memory from a reserved space, since Developers are counting on that memory for their apps.
What types of apps would even need to run at the sametime as a game? The reserve amount can be changed anytime with a firmware update. The benefit of a closed system is optimization. A app on either the ps4 or Xbone should use no more memory then an app on the IOS or Android.
Even if you wanted to watch a movie on netflix and then switch to a game then comeback to finish the movie should required little to no memory usage. the only data that needs to be saved is what movie your watching and where you left off. Its not like the entire movie needs to held in a buffer.