PS3 titles are already admitted to not be supported by PS4, although they'd try to get them playable "in some form." Their architecture is vastly different, so PS3 games would be emulated.
The trouble is they can only think of working with those PS3 titles that wouldn't suck away all system resources in being emulated, but all this is "longer term" stuff.
PlayStation 4 is expected this Holiday 2013.
Shuhei Yoshida confirms PS3 digital titles 'won't transfer' to PS4
21 February 2013 | By Simon Priest