enemies.append("res://Enemy_4.tscn")
That's adding a String to an Array of Strings.
Global.instance_node(enemies[enemy_number], enemy_position, self)
That's passing a String as the first parameter.
var node_instance = node.instance()
That's attempting to call a method instance() on a String, which has no such method.
To create an instance of a scene path such as "res://Enemy_4.tscn", you need to load it first.
var scene: PackedScene = load(node)
var node_instance: Node = scene.instance()