To simplify what jhurvid said:
The Master Sword is a key
on things (entrance to Sacred Realm, Ganon's power, etc.), and the Ocarina and Stones are keys
to things (Door of Time).
EDIT: I guess this sort of fits, but the official art for the Four Sword (the weapon) from FS looks like this:
. I guess that doesn't rule out the Four Sword being involved in making the multiple Links in the PH art.