Step-by-step run emulators from a PS4 USB Hard Drive »
At present the PC computers are not powerful enough to run an emulation of the Playstation 4 hardware. PS4 is essentially a little modified high-end gaming PC with more or less standard hardware parts. So the best bet is that there will be "wrapper" programs and drivers like we have for the 3Dfx Glide games or some soft of interpreter program that will trick the software into thinking that it is running on a genuine Sony Gaming PC = PS4 when in fact you would run it on your desktop or laptop.
Meanwhile there are good working PS2 emulators and PS3 emulation should follow shortly. Running an emulator on PS4 that would make it backward compatible with previous generation games would be a treat. This most likely will be achieved buy first booting Linux OS on PS4 and than running the emulator software in the Linux environment. Linux would also unlock the doors for all the emu software that is already out there to play NES, SNES , MAME and other classic video game roms.