What will be the temparature in shanghai , Wenzhou, Shantou,& Guangzhou in December?

At the first place, glad to tell you that these cities are all in southern China. Thus they won't be that cold. However, you may also feel a little cool sometimes.
Shanghai and Wenzhou will hold the average temperature of about 10C and the sunny days are more than the rainy days, so you can wear a sweater plus an overcoat to get through Dec. in the two cities.
For other two cities, it will be a little warmer than the former two, with the average temperature of about 16-18C. So the overcoat and cardigan can be proper.