成唯識論演祕 7.128

7.128

不取無漏智為真實，四真實體不攝道諦，故為「減」也。第四真實准第三知，故《疏》不舉。

If one does not take anāsrava cognition to be real, the substance of the four realities would not subsume the truth of the path, and thus it would be 'deficient'. The fourth reality can be known by matching it to the third; hence the Commentary does not raise it.