German Citizen living in Japan

Hello, I am a German Citizen and living in Japan (Permanent Resident Status). I am planning visiting a Japanese friend who is now living in Beijing. Period of stay will be four days, and I will be staying at my friends home who will take care of me during the stay. My understanding is that I need to apply for a visa for the four days. And for the application, I need an "invitation letter" from my friend. Could you please advice, if there is a kind a template of such invitation letter which can be used as reference? If not, could you please specify what kind of information have to be included in the invitation letter?
Thank you very much for your help!
Guido Schulte