The model was not trained in fp4. It was trained in f16 then post trained to fp4.

Also this model has very similar model sizes due to llama.cpp limitations atm so it;s unique to only this model. With a proper llama.cpp implementation, you can definitely quantize this down further

https://huggingface.co/unsloth/gpt-oss-20b-GGUF/discussions/7#6892e46687cc08d0b6275bea