Nothing stops Sony from releasing a PS4 which would technically be less ambiteous than the PS3 is today. The PS4 doesn't need to be very different from what the PS3 currently is, the PS3 IMO paved a lot of the way for a PS4.
A PS4 will probably have Ethernet, Bluetooth, Blu-Ray drive, default harddrive, etc just like the PS3 offers today, I don't think it will be the step up as it was from PS2 to PS3. A faster clocked Cell processor with more processors on chip, a newer graphics chip, more RAM and bigger harddrives, that's about it I imagine a PS4 to become at its core, though hopefully Sony will push for some virtual reality add-on hardware which may require significant additional R&D to make appealing and affordable (for PS3 and/or PS4).
Looking at the PS3 specifications it was clear from the start the PS3 was not made to make a quick profit (selling more units early on would result in higher short term costs), IMO the design is far more geared towards setting long term goals, providing good quality hardware and paving the way for new technology / better approaches.