Channel 4’s app is widely available on most platforms, but they are holding back from releasing an all4 app on the Apple TV. 

Channel 4 already have a 'Channel 4 News' app released on Apple TV and also have developed the all4 app for iOS, Android and other streaming boxes such as amazon fire tv and Roku. 

Channel 4 is ultimately owned by the Department for Culture, Media and Sport and therefore by the tax payer. They should be listening to their owners and their customers that they would like their services available on all popular platforms.