One RTX4090 with 24G gives me about 200k KV cache with Q4_K_M quant for typica coding tasks, via opencode. I use Q6_K with 60k cache if the work doesn't require scanning too many files. Q6_K is really good.
Could you shed some more light on the differences when you switch from Q4 to Q6_K? Tbh, I prefer having larger context windows since it looks like qwen just figures it out eventually. Since I noticed that it really wants to fill its context by, e.g., reading many files etc. I'd be open to experiment with smaller context windows and less quantization in my ninfer fork [0] if there's a benefit to it. But sometimes I think it's just more art that science :)
[0] https://github.com/tensorninja/ninfer-4090