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.
