It's not always easy.

On the PC it's a little easier as hardware is always abstracted, on the console there is more low-level stuff going on.
But there have been multiple paradigm shifts in AMD's GPU architectures which would translate into hardware incompatability if something was built at the metal for the chips various nuances.

With that said, on the CPU side as long as instructions aren't depreciated (Which has actually happened for example: 3D Now!) backwards comptability should be fairly easy.

Also the PS4 can't emulate PS1 games as it lacks support for CD's, it would only work if it was re-released digitally, the PS4 is Sony's first mainline console not to run PS1 games on Disc.

Now emulating the PS3 is another kettle of fish entirely, the current PS4 could do it if they took a more elegant approach similar to how Microsoft has done things with 360 backwards compatability.

Basically everything is in Sony's court, they don't seem particularly interested in backwards compatability at this stage, that might change if there is more of a consumer uproar or people start voting with their wallets, but that seems unlikely...
Where-as Microsoft needs to accellerate it's efforts and bring in Original Xbox support, they are slow moving on that front, majority of games are XBLA titles, at this rate it is going to take years to even reach half the 360's game library. :P

I would say most likely because PS5 will almost certainly use same AMD x86 architecture.

Why your head is full with thougts of Playstation everyday ?

Polaris is apparently last architecture to support GCN compatibility mode that supports PS4's original chipset, with HSA console design, linked CPU, and very high chance of picking HBM lines for GDDRs in next gen... While I doubt making it backward compatible is that difficult, it isn't out of possibility that Sony tries to get away with selling game twice by not even trying to implement backward compatibility and blame AMD for it.

If Microsoft and Nintendo both spectacularly fails instead of making returns, they will probably take that route.

I think it depends on what happens with Microsoft's console business by then. If scropio fails and they decide to leave console business and just sell games on WinStore for PC I think it is safe to say that ps5 won't have b/c and Sony will just sale remastered versions of ps4 games. Otherwise, Sony will be forced to do ps5 b/c with ps4 titles because Microsoft made it clear that they will have b/c on all their future consoles.

Possibly because the majority of threads are focused on the Playstation 4 currently due to the current news cycle focusing on the Playstation 4 Slim and Playstation 4 Pro?
Not that it matters, but if you don't like it, feel free to ignore my posts, this is a forum where people get to freely express ideas.

Polaris isn't the last Graphics Core Next derived compatible GPU.
We have Vega which is built from the same technology and Navi which will be another derivative of that.

HSA isn't a console-specific feature, it exists on PC as well, HSA has even evolved to the point where multiple RAM pools are supported.

HBM lines for GDDR5? That doesn't make sense, not sure exactly what you are attempting to convey with that snippet.

If, by the time ps5 comes out the pc is the Playstation's main competition and not the xbox which I suspect will be the case, Sony will be more likely to add ps4 B/C. Losing your previous generations library of games and having to start again is always one of the biggest points people use to favour pc. This would end that.

Doubt it. It's in Sony's best interest to sell ps4s still after introducing ps5s. We are talking hundreds of thousands of ps4s they are able to sell after introduction of ps5. Let's say 200,000 x $250.00 that's 50million in revenue. Sony knows this, and it is why ps4 is not backwards compatible.

False. At this point I'm not even sure if it's theoretically possible, much less feasible to emulate the PS3's CELL processor on the PS4's x86 processor. Sure they could easily get PS2 and PS1 emulation going (provided the disc drive can actually read the discs), but that's a different can of worms.

this.

that's the whole point of moving to x86.  and by supporting full BC and 1 gen back of FC sony is basically locking users into their ecosystem in the context of most people will have a hard time justifying walking away from their library of games.