THOMAS Cook is to expand its China operations by opening four more offices in the mainland by 1996, according to commercial director John McEwan. ''Most of our strategic planning emphasis is going into China where we will invest millions of dollars over the next five years,'' he said. The company, which has offices in Beijing and Shanghai, plans to open offices in Xian, Tianjin, Harbin and Guangzhou by 1996. Mr McEwan said the company aimed to cater for the growing number of expatriates based in China and for its travellers to the country. ''Last year our revenue from ticketing in China was HK$15 million, and this year we expect to do $30 million in sales,'' he said. ''With 50 per cent more foreign companies in Shanghai in 1993 compared with the previous year, this is our fastest growing market.'' The company will also offer its travel services to mainlanders with China Travel Service. Thomas Cook is also considering issuing yuan-denominated travellers' cheques once the currency becomes fully convertible. Last year the company issued travellers' cheques worth more than US$100 million through the Bank of China, which was the largest issue of such cheques in the country. Mr McEwan said the company's Hong Kong operations would be expanded to handle inbound passengers travelling on elsewhere. The company wanted to acquire a Hong Kong-based inbound travel agency by the end of the year.