I actually run my unit this way almost all the time when I split the map and 2D so I can see the 3 different readings with the biggest split. That image isn't a very good representation of what it actually looks like, or at least compared to how it looks on mine. 

That image was sent to me by a friend in Texas of one of the bridges on Lake Fork.

Or maybe I should ask if the 4200 will work on aluminum because it doesn't mention metal on the package

Either one works on aluminum. Use the one that is most suitable for what you want it to do.