there is no way to stick a video DVD in the navigation computer and have it play video on the screen. itd be nice if there was some hack or download to make it do that but there is nothing. Ive never checked on Z4 but I know on other BMW Models a video module is available for European models. because it is illegal in most US states to have TV in the front of the vehicle BMW does not make it an option here. Some people have imported parts from overseas in order to "retrofit" Televison and /or DVD capability howvwer it isnt a cheap or easy retrofit. if its something you want to do check BMW Bluetooth Retrofit Information
or Nav-TV™ - Car Entertainment. Watch TV, DVD, or Satellite on your factory navigation screen. Navigation TV.
and see what they have available for your model. Also you can check RealOEM.com Online BMW Parts Catalog
to see if it lists a Video module for your car.