visit xian for 4 days to attend the conference, then beijing & shanghai for tour. pls advise

I think you can visit Beijing first and then go to Shanghai after you finish your conference.
After you finish your meeting in Xian, I think you had better stay in Xian for about two days to enjoy the splendid history of Xian. Terra Cotta Army is the must seeing spot in Xian. Then if you want to know the history of Xian, I think you cannot miss Shaanxi History Museum. Besides, the Bell Tower and City Wall are all worth seeing.
Then you can take the train to arrive in Beijing. There are many places to see in Beijing and here is a list of Beijing's attractions. You can pick several of them to see. https://www.travelchinaguide.com/attraction/beijing/. Among them I think the Forbidden City and the Great Wall cannot be missed.
In Shanghai, you can choose to go to the Bund and see the Huangpu River. Yuyuan Garden is worth seeing, too.
After that, you can go back to your country directly from Shanghai.