Those are tropical and humid cities. Weather won't be much different between them. You don't need warm clothes for the summer, a light jacket/windstopper might be good.
Not sure whether you are traveling for leisure or business. You will be in 4 cities in 14 days don't need to bring lot of clothes so 3-4 set of clothes should be adequate. Clothes are inexpensive in Bangkok, Shanghai and Beijing.
You didn't mention whether you are male or female so clothing needs will be very different
Try to to search the web for "travel packing list" etc.
http://travelindependent.info/whattopack.htm
Thanks for the link. I'm a guy and Thailand will be for vacation and the rest of the trip will be for work. However, work is casual for me so jeans/polo shirt will be fine.
Based on past packing experience, I think I should be able to get 4-5 outfits in a carry-on.