The way Sony has done all their consoles up to now, the PS4 will be out about five years time. They make a new console every five years, and each one designed to last ten years (the PS1 has been almost completely phased out now, and the PS2 will begin its decline in another year or two). It's a little early for rumours of a PS4, especially when Sony have such a battle for the current generation ahead of them already...
As for the next PlayStation-branded console, my money's on a PSP2 - complete with touchscreen