大般涅槃經義記 4.302

4.302

四明說所因。「欲受第四八戒齋」者，有言於彼明六齋中受第四齋，亦可於彼小乘法中有二種戒：一者盡形、二者日夜。盡形戒中有其五戒、十戒、具戒，約對彼故，宣說八戒以為第四。

Fourth, he clarifies the reason for speaking it. Regarding "wishing to receive the fourth, the eightfold fasting precepts," some say that this refers to receiving the fourth fast among the six monthly fast days; it can also be that in the Hīnayāna Dharma there are two kinds of precepts: first, lifelong; second, for a day and night. Among lifelong precepts there are the five precepts, ten precepts, and complete precepts; in contrast to these, he proclaimed the eight precepts as the fourth.