ووجه الانحصار في الأربعة على ما ذهب إليه المصنف ظاهر
The reason for restricting the division to four cases is clear on the adopted definition.
Each of the two sides has two lexical possibilities: literal or figurative. Two binary choices yield four combinations.
The predicate under discussion is an action or action-meaning. If one changes the scope of the definition to include further kinds of structures, different classificatory questions can arise. The apparent fourfold restriction depends on preserving the original scope.