In Metric Insights v.4.0 we moved away from conception of linking multiple Reports/External Reports to one Metric Alert rule. Now we can choose only one element to be linked with one Alert Rule.

On the screenshot you can see Send with the alert drop-down where you can select Other Visualization and choose Report/External Report/Metric from the list that would be sent to your email when Alert Rule is triggered.

Note. If your Metric with Alert Rule is dimensioned, you can choose visualization from elements that are not dimensioned or dimensioned by the same dimension as this Metric.