Depends on time of release. I'd guess some 17 to 18 GB, with 1-2GB dedicated to system functions and background tasks.
A later release could lead to 33-36GB with 1-3GB reserved for system operations. I don't expect this gen to go on as long as the last.
Also
PS1 2MB RAM (system)+ 1MB VRAM
PS2 32MB RDRAM (system)+ 4MB eDRAM
PS3 256MB XDR (system)+ 256MB GDDR3
PS4 8GDDR5 (unified)+ 256MB DDR3 (for background task)
EDITED.
http://www.redgamingtech.com/ps4-developers-were-speechless-when-8gb-gddr5-ram-was-announced/
Last edited by Dgc1808 - on 15 October 20184 ≈ One