Quaternary means four, in this case it describes a four-cornered Celtic knot design. According to George Bain, a Scottish Celtic art professor, Celtic knots with a closed path are a symbol of eternity, or the infinite. Where as those with open paths signify a journey both physical and spiritual.
The Quaternary can represent the four Latin Gospels in the Book of Kells, the four cardinal directions: north, south, east, and west, or even the four elements of earth, fire, water, and air. As with most celtic symbols there are multiple meanings based on your point of view.
The pendant does come with a chain, but the loop should fit any 1.8mm diameter chain.