Our understanding of these terms needs to be re-evaluated.
Most of us sort out actions according to these categories and stick to the faraz & wajib only. We consider sunnah & mustaheb to be optional choice reserved for the committed elite.
This is totally against the Ahl-us-Sunnah methodology.
The classification of actions by the fuqaha as Faraz, Wajib, Sunnah and mustaheb is late development in the Islamic history. These terms were not used during the period of Sahaba and the ta’baeen. They just copied the actions as done and reported by the Prophet salalaho alehey wa alehei wasalam (Sunnah).
Kisi ko dekhtey rehna namaz thi meri
However, with passage of time a system evolved to assess the acceptability of actions tainted by omission or negligence. That is, to ascertain either these deficient actions are valid or they have to be repeated again. It is then that the fuqaha developed these categories. They never meant to propagate the bare essential compulsory acts, disregarding & discarding the Sunnah.
We need to correct this concept and correct our actions accordingly.
(This is a synopsis of what I understood from a talk on this topic by Sidi wa sanadi Moulana Mohammed Taqi Usmani sahib db)