AFAIK it has shader binary compatibility so it should "just work" but performance may not be great without a patch.
Sure - just like Series S can run all the Series X games as-is, just performance may not be great if they don't support it explicitly... Oh wait...
The whole 'it needs a patch to perform' means it won't run games out of the box, period. No amount of semantics debates about ISA compatibility changes that - hell PS1 and N64 technically shared the ISA with the PS2 - doesn't mean they were forward compatible with PS2 games.
Was DF saying even to make a portable PS4 is a challenge?
Define 'challenge'.
Modern AMD APUs can run PS4 profile at 15-20W just fine(often with improved performance0. Yes you'd need a special memory subsystem to get full compatibility(not the crap they put into existing portables, steam-deck included) probably using eDram, but that's been par for the course for consoles for decades. Unlike PC in a box 'closed systems' - they actually have memory subsystems designed for the chipset.
I would be very happy with a portable PS4 to be honest
You and the 5 people that bought a PSVita too. But if Sony is seriously making this thing, they'll need more than 6 people to buy one.
Is there any sight of use of some AI?
I hope 'someone' shows AI use that isn't just 'hey, there's the same exact game that you've been playing for past 25 years, but now with slightly enhanced AI pixels, and even inserting extra made-up AI pixels, but all importantly - it's AI enhanced pixels, because that's all we can do'.
But that won't have any relevant impact on BC with PS4 or PS5 (and it shouldn't - they really need something that changes the paradigm for a change again - it's been 20 years since the last shift).