In your case, it is fairly easy: do not think that is an isolated entity. It comes with , with ``, meaning `The fourth`.
In this case, whenever you encounter a counter, go with the . This was highlighted in the answer you linked:
> Kunyomi is typically used in cases where you point out you have X of some item.
For example, you would say to count flat things, to count cylindrical things, which is another way of saying fourth, to count animals and so on...