Lmsys hasn't released any official weights for anything. They've released "deltas" and other people have applied those deltas to the appropriate llama weights and done the quantization.

I reject your premise that the 8 to 4 bit quantization is the cause of the vicuna fine-tuned llamas very average performance though. This hasn't been the case for any of the other 8 to 4 bit quantizations. It would be a unique outlier. And so I don't think this is the "cause" here.