By April temperatures have improved somewhat in the north and in Beijing. Beijing is warm, windy and dry at this time of year, making it a good time of year to be in Beijing.
A light jacket is still a good idea as temperatures can feel quite cool at night. Flowers are in bloom and there can be occasional rain or the odd sandstorm.
In Shanghai the daytime temperatures are usually pleasantly warm, with cooler mornings and the possibility of occasional rain. Average temperatures in April in Shanghai are 20 degrees.