Vitthāruddeso catuttho is the second last line of the bhikkhuni patimokkha. What does catuttho (the fourth) refer to? What are the other three things?
It refers to the fourth uddesa, the nidāna being the first, the pārājikas beings the second, and the saṅghādisesas being the third. Vitthāra refers to the remainder of the pātimokkha. It seems to be a division unique to the commentaries.
Thanks, Ajahn, that clarifies it.
It seems that the bhikkhu pātimokkha has five uddesas then? One additional for the aniyatas?
And that the bhikkhu pātimokkha is missing Vitthāruddeso pañcamo at the end is just a quirk in the history of transmission?
Yes and yes. The VRI chaṭṭasaṅgāyana version actually has vitthāruddeso pañcamo at the end of the bhikkhu-pātimokkha. So things are bit random as usual!