Sony won't be abandoning all its hard work on the PlayStation 3 just because PS4 is launching later this year.
If you remember way back to 2006 the PS3 launch price was $499 and $599, significantly higher than the current $399 for the PS4. Sony looks to give consumers some longevity out of their purchase.
Sony has a history of supporting consoles after they've effectively been replaced. The PlayStation 2 continued to have releases and became an indie platform years after PS3's launch. Though consumers are encouraged to upgrade, keeping around the old box has some advantages. Many new releases are still planned as dual-launches and users will still be able to play online, though not free anymore. In 2014 Sony will launch its cloud service in the US allowing users to play PS3 games on the PS4 but Sony looks at the next-gen switchover has a gradual process.
Sony's Hiroshi Kawano and Shuhei Yoshida summed it up nicely in an interview with Weekly Famitsu, "to say 'we're releasing a new console, so trade in your old ones for it' would be a maker's ego talking, plain and simple."