There IS a 4k 40 fps mode.
But for whatever reason, the game simply does not scale well on the PS5. Even the 4k 40 fps mode dips all the way down to 32 fps and averages around 34-35 fps during combat. It's 40 locked only during exploration. Im guessing they had an engine bottleneck for the CPU. We have seen this with PC ports of Horizon and GOW which are single threaded and hold back even the most powerful GPUs, let alone PS5 equivalent GPUs like the 2080. The GOW PC port team even admitted that changing the CPU tasks to be more multithreaded wouldve required a lot more work so they left it as is. We are probably seeing something similar here.