The Octo-core inside the S4 is not really an 8-Core CPU. It's two Quadcores literally duct-taped together, and there is always only one of them active. Besides, Europe gets "only" a quad-core, but with 1,9 GHz.
That said, for emulation of the psp Smartphones would need much stronger GPUs. While a strong CPU sure helps, there are just operations that need a GPU to be processed efficiently. It is simply not possible with 100 % speed and sound at the moment. But even if it eventually will become a possibility one day, the battery would die fast as shit, because all components of the smartphone would run at full speed. So you're looking at a damn expensive piece of hardware that runs PSP Titles for like 30 Minutes. I think just buying a PSP or a Vita would be the better option. Even more in times of Smartphones without changeable batteries.
Official member of VGC's Nintendo family, approved by the one and only RolStoppable. I feel honored.
