Question on extension of a tourist L Visa.

Hello I am an American citizen who wants to visit China for 4 months. If I acquire 3 month tourist L type, can I get extension of the duration of stay at the exit & enter department of local public security authorities by 100% chance? If not, what can be the reason? In addition, can I visit South Korea near the expiration of 3 month L type, and then reenter China in order to stay one more month? If yes, I want to know how. Thank you and I appreciate your forum.