With an understanding of the console architecture in question, it doesn't look so easy. The ps2 and ps1 should be penuts to emulate (it is on PCs weaker than the PS4) But while the Xbox One and the Xbox 360 both have architecture much like a PC, it's a different story over at Sony. I'll try to explain.
PS4 has very PC-esk architecture as well (Xbox One and PS4 are very similar in that respect) but the PS3 had a very irregular architecture called the Cell Microprocessor which splits its processing load across one main processing unit and six additional co-processing units.
PS3 was notably difficult to code for, because not only did you need to keep all six of these co-processors busy, you had to keep them precisely equally busy at all times, or the game's systems would fall apart. It was very fiddely.
Emulating the PS3 is extremely difficult because of this very esoteric processing setup. Without that architecture (which is very different to what the PS4 is packing) it needs to be run more or less entirely through software, and even computers superior to the current consoles aren't juiced up enough to be able to handled an entire last gen console running purely on a software level.
Long story short, I'd love to see the PS4 successfully emulate the PS3, but it's not at all likely. PS3's bizarre hardware design has shot Sony in the foot in that aspect. We're seeing early Xbox 360 emulation happening on PC, but we likely won't see successful PS3 emulation for a while. Although I do recall murmurs that somebody had managed to make one that could only play one game (I forget which) and reported that emulating a system for that one game took the same amount of work that it usually takes to emulate an entire console.
I can't explain beyond that point. I understand the architectures, but I have never contructed an emulator myself. But that's my understanding of the situation. Ask around, but the concensus is that that damn Cell Microprocessor is a massive road block.
