I'd agree that the quality degrades a lot between Q8 and Q4, borderline unusable as they start to fail with tool calling syntax even. Personally I'd say Q8 is as low as you want to go.
q4 isn't rubbish, but it's a compromise for a good value, q6 is essentially a no-compromise quantization and it's what i recommend for MoEs in my experience for agentic workflows