As per an internal document handed to Digital Foundry by an unanimous source, the PS4 reserves 3.5GB of its 8GB GDDR5 memory for the operating system, leaving around 4.5GB for game code. This, coupled with an additional 1GB of flexible memory, brings up the tally to 5.5GB for code.
As revealed in the documents, the 4.5GB, which is guaranteed, is “the baseline amount of guaranteed memory available for game-makers and most likely what the lion’s share of launch titles will be using.”
When we take the devkits under consideration, the normal mode shows usable memory of 4.5GB, while the large mode increases it to 5.25GB. However, the extra RAM is for “application development only”.
A lot of technical stuff, this. If you want the full report, go through the link provided here.