成唯識論演祕 6.150

6.150

答：有第四業必四業俱。不言恒時皆四業並。故無其失。若恒時俱但前三業。

Answer: When there is the fourth function, the four functions must be concurrent; it does not say that at all times all four functions are aligned. Hence there is no error. If it is what is constant, there are only the first three functions.