Yes you cannot play PS3 games on PS4 you have to purchase PS+ I believe and this give you downloadable PS3 games. They are adding games as times goes on.
You maybe thinking about the streaming service which isn't available at launch but it is rumoured around 3,000 titles will be available.
Not a great deal else has been said about it or WHAT titles will be featured or WHEN this feature will come live.
I think this is what your looking for, the service is called Gaikai.