《明史·志·卷六十五》
于是征集九边的突骑家丁几万人到京城，称为外四家。
于是集九边突骑家丁数万人于京师。名曰外四家。
Thus, several tens of thousands of cavalrymen and armed retainers from the nine border regions were recruited to come to the capital, and they were called the Four Guard Units.