What is Alius Cantus Aptus?

Here is a very interesting argument that says GIRM's option four, to sing another suitable song, doesn't mean: sing whatever you want in place of propers. This author argues that, in context, option four is actually suggesting that we sing another suitable setting of the proper text. This does indeed help to make sense of the rules. After all, it is a bit strange to have three very specific rules and a fourth that seems to negate the whole point of having any rules at all.

"What makes a setting suitable is its use of the designated text."

I find the argument intriguing. It sometimes helps to think "outside the box" on these matters. It very well could be that the option 4 proviso has been widely misunderstood.