How? Not at all. That is How .

You don't really need emulation for backwards compatibility.

Furthermore, speed of a cpu doesn't isn't a direct correlation with power. Rather the issue is compounded by the difference in ISA. But again, emulation isn't a hard requirement for backwards compatibility if you utlize the network ala XB1.

They should have an add-on of some sort for BC. I would buy it and sell my PS3. I don't think it would decrease current gen sales as there are many like me who just kept their old system if they really wanted to keep being able to play old games.

I've thought of that, but the issue is thats basically just a PS3, granted a more convient PS3 that connects to your PS4. 

they could at least use the PS4's GPU and RAM.  I think the Cell and some bridging hardware could keep it relatively small.  Also no disc drive since that's already provided.

As I said in the "What would your reaction" thread BC is most definitely possible on PS4.

I see a common thought process within this thread and anywhere where this subject is brought up, where people are assuming that all of the Cell's code would all have to be emulated by PS4's Jaguar cores, that isn't the case at all. For one thing SPEs aren't CPU cores, so of course trying to run the code SPEs handle in PS3 would be a futile endeavor on PS4's CPU, it would be futile on any CPU, but given that Vector math is handled by PS4's GPU it makes a lot more sense to move these tasks over to PS4's GPU, something that a PS3 virtual machine can be designed to do.

It's the combination of PPC and SPEs in one unit that is the issue here, technically PS4's APU has the technology needed to process all of the math both of those devices handle within it's SOC.

Microsoft have already shown that 360's PowerPC tasks can be emulated by the Jaguar CPU in XB1, PS3's Cell only has a single PPC unit, so it's actually a much more simple thing to run PPC code on Jaguar and SPE code within a portion of PS4's GPU.

RSX is relatively weak compared to PS4's GPU too, so emulating that on a modern, substantially more powerful AMD GPU is definitely doable.
A translation program built into a PS3 virtual machine could translate RISC and ISA language to CISC for PS4's technology to be able to recognize it.

A virtual machine would be designed to move necessary processes to where they make the most sense to process, it's most definitely not an impossible thing to do, just complicated.

As for whether it would be worth it, of course it's worth it, it would mean PS3 games could be sold on PS4, Sony can market PS4 as a system capable of Backwards Compatibility in PS4.
PS Now is a subscription service and it requires a permanent internet connection, Backwards Compatibility would appeal to the audience that doesn't want to require a constant internet connection to play PS3 games through their PS4.
So BC would only require an intermittent internet connection for when you download the Virtual Machine and the odd patch wrapper per game you're yet to run on your PS4.

Sony could tie BC to Playstation Plus as another incentive for the service and just like Microsoft does any game that is added to the Instant Game Collection for PS3 games could be played on PS4.
More sales of PS3 games and potentially increased numbers of PS+ subscribers could more than make up for the costs of making a Virtual PS3 to work on PS4.

GPUs are fast at calculations but not that good to replicate a data flow that happens on a CPU. CPUs do a lot more than just "add x to y and save it in z", especially memory operations and ram access is crucial.

The Jaguar of course is somewhat "modern" but it is still way too different to Cell than the Jaguar could handle it. You still would need several clock cycles to do what Cell does in some ops which immediately shows that the Jaguar isn't fast enough - it would need to do several Cell instructions in one instruction to make the clock speed difference irrelevant.

Whether you are proven wrong or right is of no use here. PS3 emulation won't happen on PS4. If you want to take a look at a PS3 emulator, look up for rpcs3. There are even videos out there of it running, many of them show the task-manager so you can see the load on the machine that is emulating. And you can see the machine specs that is emulating. It easily wipes the floor with the PS4 in terms of CPU.

I would say no, remastering games as of late has become a good way to make sales on a old game, plus the game is remastered for the new gen console. Just looks better.