I would point out that our precedents in Swift with respect to words that function as binary operators (as, is, in, etc.) are all multi-letter (and most if not all are two-letter words). *
That the Rust folks proposed [4 of Int] until it was realized that they had Rust-specific parsing issues with it gets us closest (I think) to something of a reassurance that it's not a bonkers spelling.
* And if we really want one alphabetic letter only, we can consider the delightful o'—[4 o' Int] has a jolly lilt to it that rolls off the tongue 