For what we know now, the Ps4 is not backwards compatible with any Ps3 title, whether it's digital or in physical medium. That could all change with their new streaming service called Gaikai that it set to be launched in early 2014 in the U.S .
hi op what others said is correct but here is more infomation on the service that sony going to have in on ps4 that will alow to play older games. here the link to it http://en.wikipedia.org/wiki/Gaikai . i hope this helps tc and have fun.