Apparently they were ordered specifically for the Green line and it's extension. When they're all in revenue service, all the 4k trams will be moved to the Red line to service it and it's two extensions.
They're pretty much identical - just 3m longer, slightly different seating configuration and step-free aisle along the entire length. They also have a louder bell that seems to trigger a spree of bell ringing from all the drivers and an ugly snout on the end.