I would imagine the PS4 would be using a SSD for performance reasons, although maybe not since they are significantly more expensive per GB than a HDD. But maybe in a few years, when the PS4 is ready to release, SSD's will be much cheaper. Worst case scenario is that it will be fully SSD-capable, meaning it has TRIM and all that other stuff to reduce the wear on the drive in case somebody wants to use a SSD. Cause while you can use a SSD in the PS3, I don't think it has TRIM capabilities (maybe it does, anybody know?)
One thing we can count on is that the PS4 will no doubt will be touch-technology ready, because that seems to be the way things are going nowadays (like with PSVita and Windows 8 for example). Not something I want to see happen, but that is the demand. I would also think that the PS4 will have to think ahead and up the maximum resolution beyond 1080p, because we are reaching the time now where 1080p isn't good enough for home HDTV's, and pretty soon we will start to see HDTV's that will go much higher than 1080p.
Either way it seems like the PS3 is finally starting to get up to speed and it still seems like developers haven't quite yet gotten all they can out of the PS3's power yet, plus it just feels too early for another one. Maybe in 2-3 years, but 2012? No way! In order to make something significantly better than the PS3 (which is what each new generation console aims to accomplish), the price tag would be enormous. In 2-3 years that technology will have dropped in price drastically and would be more reasonable. But not as early as 2012.