ESD protection Touch screen
You can't do much in improving the ESD if you are buying the screen from the vendor and not designing it yourself, if you have a connector in your PCB,the most you can do is add TVS or ziener diode between the connector pins and ground, diodes are only added on digital lines, for supply and analog line you will need to add a 10nF capacitor, the capacitor should be 100V rated, and should have a size of 0603 or 0805 the bigger the better.

As for the metal box, as it is metal, it is susceptible to ESD so you should make sure it is connected to ground. 

I can't help you more on this as i said if you don't design the screen you can't do much, also i don't have the schematic, for all i know, you might have made all the protections proposed as you have passed 4kV ESD.