The full fourth tone marks the noun. 公道 is justice, something you demand or uphold. Said with a neutral tone, 公道 is the adjective fair or reasonable, used of a price or of how someone acts.
usage
This reading lives in a few fixed combinations: 讨公道 to demand justice, 主持公道 to see justice done, and the saying 公道自在人心, that people judge for themselves what is fair.
