Relation pm#quaternary_relation__quaternary_only_relation (?,?,?)  relation with a fixed arity equal to 4
  exclusion:  pm#binary_relation  pm#ternary_relation  pm#relation_with_variable_arity
  supertype:  pm#relation_with_particular_property  this rather fuzzy type permits to group categorization schemes less common than those covered by the previous sibling categories
  subtype:  kif#subst (?,?,? -> ?)