4 bit is as low as I like to go. There are KLD and perplexity tests that compare quantizations where you can see the curve of degradation, but perplexity and KLD numbers can be misleading compared to real world use where small errors compound over long sessions.

In my anecdotal experience I’ve been happier with Q6 and dealing with the tradeoffs that come with it over Q4 for Qwen3.5 27B.

Generally the perplexity charts indicate that quality drops significantly below 4-bit, so in that sense 4-bit is the sweet spot if you're resource constrained.