1 is about object identity. 4 is about object type/function. I.e. polymorphism. It may seem like a weak criteria if you're using to OOP/FP languages that provi

1 is about object identity. 4 is about object type/function. I.e. polymorphism. It may seem like a weak criteria if you're using to OOP/FP languages that provide typesafe, efficient polymorphism out of the box. But the reason we have Obj C and C++ is that polymorphism (while also maintaining encapsulation etc.) is a living nightmare in a language like C.