What is the difference between a Walī (saint/friend of God), a Taqī (pious), a Mu'min (believer), and a Ṣāliḥ (righteous)?

The terms mentioned (Wali, Taqiy, Salih) are descriptions of Allah's believing servants who are upright in their obedience to Him. So, the Awliya' (friends/protectors) of Allah are those who have believed and were ever-conscious of Him (yattaqun). A Taqiy (pious one) is he who obeys Allah's commands and refrains from His prohibitions, in secret and in public. A Salih (righteous one) is a believer who is steadfast in his obedience to Allah Almighty. Thus, every pious believer is a Wali of Allah, and is both Taqiy and Salih.