The Heritage Illustrated Dictionary of the English LanguageInternational Edition  in its Usage note for should includes the following statement Should in indicating obligation or necessity is somewhat weaker than ought and appreciably weaker than must and have to As I understand these four expressions they all can denote obligation as when an army officer commands a soldier You should intercept this tea You ought to intercept this tea You must intercept this tea You have to intercept this tea Also they all can denote recommendation as when a person speaks to a friend You should try this tea You ought to try this tea You must try this tea You have to try this tea Intermediately they can denote quasiobligation or quasirecommendation as when a physician issues a command or a recommendation to a patient You should try this tea You ought to try this tea You must try this tea You have to try this tea Also they all can denote probability as when a person expresses a conclusion about what seems to be likely and not necessarily desired This tea is very good so it should be very expensive This tea is very good so it ought to be very expensive This tea is very good so it must be very expensive This tea is very good so it has to be very expensive In my understanding the four expressions differ in their intensity but their meaning varies according to their context