I would list the four scalars if I wanted four scalars. You can always define the format of @coordinates at the top and use that, but I don't see any gain in doing so.

The name of a variable can only tell so much. Extra limitations (e.g. that it must be an array) can be documented below.

I expect you now to say that you always avoid prototypes, right? ;-)

Close. I tend to write classes, and prototypes don't apply to methods.