You are right, Beijing is in northern China, Xian is at central area, Shanghai is in the east, while Hk is southern Island. The weather is different between them. In early April, Beijing is still a little bit cold, with an average temperature of 14C. You have to wear wool a sweater and thick overcoat. At that time, Xian is warmer and sunny, trees will bud and flowres are or are going to blossoming. A long-sleeved T-shirt plus a light overcoat will be enough. In shanghai, the temperature is 11-19C, dry and warm. Long-sleeved shirts, thin sweaters or coats are suitable for this time. The temperature in Hk then will be 20-25C. It is warm and humid, accompanied by fog and drizzle. So long-sleeved T-shirt plus a light overcoat will be ok.