First of all targeting 3200x1800 when frame drops to 37 is retarded. Should opt for 2700x1500 which is basically twice 1080p and half True 4K and then do checkerboard rendering, performance already should be better like that. If frames are still below 60 constantly on 1500p then it was a mistake.
Killing Floor is a game with a massive amount of enemies. The game is most likely CPU limited when it drops
This is quite likely. They are barely more than an indie dev (started out doing mods out of their homes for Unreal Tournament), and are probably not the best indication of anything on PS4P (or competent/budgeted enough to deal with great optimization for the garbage AMD Jaguar compared to Intel PC CPUs).
It reminds me of when everyone was freaking out over how god-awful that Sniper game was on X1 back in the day. It had atrocious tearing and optimization, but at the end of the day with evidence like FH2/FH3/Gears 4 and even Ryse, the X1 was capable when matched with capable devs.
Tripwire Interactive is ... well, I will be nice and say they try hard with limited resources.
As with anything with these limited consoles (even PS4P or supposed Scorpio specs), it will be a mixed bag. They are hugely compromised even for today.