والنفل والتطوع نظيران
Nafl and tatawwuʿ are comparable terms.
Tatawwuʿ emphasizes willingness and voluntary devotion, while nafl highlights that the act goes beyond the principal obligations. In this discussion, both point to the same general legal level.
This does not eliminate the separate classification of Sunnah. To identify nafl within the fourfold scheme, ask: is performing the act binding? No. Is performing it recommended and rewarded? Yes.