Wild speculation time!

My bet is that Level 4 Skillchains will come from chaining Weapon Skills with Light and Darkness elements. I believe that is currently limited to all Relic and Empyrean (but not Mythic) Weapon Skills, with the exception of the Staff Empyrean Weapon Skill Myrkr, and the addition of the Sword Campaign-only Weapon Skills Glory Slash and Uriel Blade.

I am also willing to bet there will be a set of lv.99 Weapon Skills with Light and Darkness elements. I give even odds on them either coming from skill or being quested.