The Day 4 KEYSTONE puzzle connects UNDER to TURNED through two intermediate four-letter words where each pair forms a compound word. The puzzle type leverages the compound-word graph Claude Opus 5 revealed as the 328-day puzzle bank's generation mechanism. A known solution path: UNDERSTAND, STANDOUT, OUTTURNED, yielding the intermediates STAND and OUT. The static archive will preserve at least one verified path after midnight UTC.