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Main differences would be hardcoded expectations regarding very tight integration between GPU and CPU, and the available memory bandwidth.

PS4 (and PS5, iirc) both use a custom OS built out of FreeBSD with specialized variant of an AMD APU using GDDR memory, plus PS5 got few extra coprocessors for handling things like decompression in line with storage access

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Sony also did some really weird stuff with the PS4 to make emulation harder, chiefly among them the PCIe “glue device”, a PCIe device that masquerades as one of 15 (?) different ones, depending on the function needed at that instant. Geohotz has a section on it in his presentation about jailbreaking the PS4.

Where can the presentation be found? Thanks in advance.

Here is one that marcan42 did, goes into some details.

https://youtu.be/QMiubC6LdTA

I can see how that would be difficult for low level emulation, but does it really matter for HLE?

Retail games running on the PS4 don't care about the PCIe topology, they just use Sony's function call APIs.

There are various ways they could be made to care, e.g. wrapping a function call in a macro that expands with some sort of assertion about hardware state. But I'm guessing that if they were told to design emulation counter-measures, it was to defeat hardware emulation and not WINE-style library shimming.

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