What is the weather generally like in Beijing, Shanghai, Suzhou and Hangzhou in September?

Billy, it's the beginning of autumn in September. Compared with previous months, the temperature drops down. More importantly, it's sunny but not very hot most of time. So it's time to travel.

Beijing:15 to 26 Celsius Degrees
Shanghai and Hangzhou: 21 to 28 Celsius Degrees
Suzhou: 19 to 29 Celsius Degrees

At daytime, you can wear T-shirts, shirts, thin jeans. At night, you might add a thin jacket as the temperature is down.

September is a very good month weather wise. But it can be chilly early in the morning and late at night. The kids go back to school in early Sept, just like in the USA, so traffic/train travel can be a problem at that time.