I am a South Korean, and want to visit China for four days. What should I do to get a tourist visa?

Do you mean you want to apply for a China tourist visa in Italy? Besides the completed visa application form V.2011A and V.2011B, you also need to provide your passport, one passport photo, the Italian residence permit or other document to prove your legal status in Italy, maybe the round-trip flight tickets and hotel booking are also required.

Thank you for your quick answer. I have another question. I am planning to visit China in December.
Could I apply for the tourist visa NOW even if I will visit China for four days in the middle of December this year? Or should I WAIT and apply for the tourist visa a little bit later because I still have three months before I visit China...I am worrzing about the duration of my visa might expire.

A single entry visa is usually valid for three months since the issuing date. So you'd better wait a little later and go to apply 1-2 months (at least two weeks) ahead of the entering day.