Sony owns the Gaikai game streaming service, and there are a lot of rumors that they'll use it to allow people to play (via stream) old games. Sony has released statements saying basically that they're trying to do that, although it may be further in the future. So. . .yeah. There's a good chance you'll be able to use your PS4 to play old games, but the experience won't be as good (streaming has an inherent latency issue), and you'll no doubt have to re-buy the titles.