DLSS3 never was "4000 exclusive", only the FG part was (and still is, now for 40+50).
The problem AMD faces with FSR4 is two fold:
1. They need to port it to the assortment of their previous gen GPUs most of which have varying feature support for 8/16 bit matrix math.
2. They still sell these parts (and even launch new ones on RDNA3! CPUs also have RDNA3/3.5 in them) which means that running a more complex upscaling on them will make them perform worse than they do now with FSR3. This will affect their selling power and may result in a need for another price drop.
That depends on a few things.
Currently FSR3 needs a higher quality native render to produce higher resolution with higher frame-rates on RDNA2/RDNA3 .
So provided the performance saving(in milliseconds) in dropping to much lower native and the effective gain in upscale quality using FP16 instead of INT8/INT4 in a hybrid FSR4 could offset the multiple fold loss in throughput of FP16 compared to INT8/INT4 for upscaling, then the overall outcome could still be superior compared to FSR3, and this theoretically could be true for the PS5 if the register memory from the 36CUs was adequate.
As an example, say the PS5 with FSR3 runs 60fps with native res of 1920x1080 and takes 2ms. Provided an FSR4 implementation on PS5 using FP16(RPM) can do a FMA per Half Top (so 60 TOPs = 20HTF/s x 3 Ops), then the difference between the Pro targeting 4K60 from native 1080p60 with 300 TOPs(as 2x FMA per INT8/FP8) in 1 millisecond vs 5x less TOPs (300TOPS vs 60TOPS) if the PS5 targeting a quality upscale of 1080p60 from - say - 960x540@60 could free up (5x) 5 milliseconds.
Obviously the end result would need to be a superior fidelity 1080p60 via FSR4 vs 4K60 via FSR3, and I've completely ignored the PS5 Pro custom instructions to halt and switch from rendering to upscaling, meaning the x5 might be 1ms more.
Similarly PCs with RDNA2/RDNA3 GPUS they will need to do something similar with the GPU's LLC being the other factor after the TOPs comparison and still hope on balance the fidelity is superior to FSR3 from lower native res and lower output res.
Edit: maybe I've mixed and matched Ops/sec with Pro vs Ps5 calc comparison and it still needs multiples more, probably needed a drop to 30fps too.