Sze Yup, Sze Yap, Seiyap (Chinese characters: 四邑; Cantonese Yale: Sei3 yap1; meaning four counties) or Si Yi (in Mandarin) refers to the four counties of Xinhui, Taishan, Kaiping and Enping in the Pearl River Delta, in southern Guangdong Province, China. Xinhui is a city district and the other three are county-level cities, all four belong to Jiangmen prefecture administered from the city of Jiangmen. Since Heshan became governed by Jiangmen in 1983, Wu Yi (Chinese characters: 五邑; in Mandarin; meaning five counties), which refers to all the five counties of Xinhui, Taishan, Kaiping, Enping and Heshan, has become an official title, and is widely accepted by the local residents today. However, among overseas Chinese, the title of Sze Yup is still popular and frequently used.

The area gave rise to the Sze Yup dialects (四邑話) of Cantonese. Taishanese is an example of Sze Yup dialects. Although Sze Yup and Standard Cantonese are both dialects of Yue Chinese, Standard Cantonese speakers can not easily understand Sze Yup dialects. Therefore Sze Yup emigrants have kept their identity, even among other Cantonese emigrants.

In the 19th century, many people from Sze Yup emigrated to Hong Kong, Southeast Asia, Australasia, North America and South America. The Punti-Hakka Clan Wars also erupted in these counties during this time.

Reference:

http://en.wikipedia.org/wiki/Sze_Yup