There are now more quantization types in llama.cpp, some lower than 4 bits.
Currently these are not well supported because of technical reasons.
If you want to use them, you have to build llama.cpp (from build 829 (ff5d58f)) with the
LLAMA_QKK_64 Make or CMake variable enabled (see PR
#2001).
Then you can quantize the F16 or maybe Q8_0 version to what you want.
Coming soon...