Every 4K external display I've connected to every M1- and M2-series Mac running macOS has a known flickering issue with Display Stream Compression that Apple knows about and has been unable or unwilling to fix.
The only reliable fixes are to either disable that DisplayPort feature if your monitor supports it, or to disable GPU Dithering using a paid third-party tool (BetterDisplay). Either that or switch to Asahi, which doesn't have that issue.
The issue is common enough that BENQ has a FAQ page about it, which includes steps like "disable dark mode" and "wait for 2 hours": https://www.benq.com/en-us/knowledge-center/knowledge/how-to...
One of my external screens is 4k and I haven't noticed any flickering. It's an Apple monitor though, so maybe that's the difference.
I've used ASUS 4K monitors on M2 and M4 machines without issues.