Add jan v1 4b config

[Tobivi]

Hi GaiaNet Team,

This PR adds a new node configuration for the janhq/jan-v1-4b model, using a Q4_K_M GGUF quantization.

This contribution includes:

• The config.json file pointing to the model on the Hugging Face Hub.
• A README.md file with clear setup instructions for other users.

I have tested this configuration on my local node and it works as expected.

Thank you!