private static void verifyProperty4(RBTreeNode<?, ?> node) {
        if (nodeColor(node) == Color.RED) {
            assert nodeColor(node.left) == Color.BLACK;
            assert nodeColor(node.right) == Color.BLACK;
            assert nodeColor(node.parent) == Color.BLACK;
        }
        if (node == null) return;
        verifyProperty4(node.left);
        verifyProperty4(node.right);
    }