What is the difference between Sunnah, Mandub, Mustahabb, and Nafl?

Some scholars hold that the terms: Sunnah, Mandoub (recommended), Istihbab (preferable), and Nafl (supererogatory) are synonymous, all referring to actions for which one is rewarded for performing them and not punished for neglecting them. Whereas others differentiate, considering the Sunnah to be what the Prophet, peace and blessings be upon him, consistently performed or made apparent, while the Mandoub, Mustahabb, and Nafl refer to actions he performed but did not consistently uphold or make apparent.