Blizzard has ensured that there will be no spoilers for how to find new Runes in the Season of Discovery Phase 4 PTR — all we'll know is the names and Rune abilities!
Players have been incredibly vocal about their desire to enter Season of Discovery phases with as few spoilers as possible. Part of the fun, after all, is in the discovery — and many were worried that a PTR would ruin that fun for the much-anticipated phase launch. This worry is unfounded, however — at least in the case of Runes.
While players can check their Rune journals to see many of the new Back Runes for their classes, along with the effects they will have, not all Runes are implemented yet, so check back in frequently; we'll have the latest and greatest Rune information as soon as we find out more!