pm#quaternary_relation_type  relates four items
  exclusion:  binary_relation_type  ternary_relation_type  quintary_relation_type  variable_arity_relation_type
  supertype:  relation_type  there are three kinds of relation(_types): pm#predicate_type, pm#function_type and sumo#list; both predicates and functions denote sets of ordered n-tuples; the difference between these two classes is that predicates cover formula-forming operators, while functions cover term-forming operators; a list, on the other hand, is a particular ordered n-tuple
  instance of:  class_of_inheritable_relation_type
  equal:  quaternary_relation (pm)
  subtype:  quaternary_predicate_type  class of predicates that require four arguments
  subtype:  ternary_function_type  class of functions requiring three arguments

No statement uses or specializes quaternary_relation_type; click here to add one.