You are suggested to go to the Apps/ Navigation Ball/ Settings. And click this "Enable Navigation Ball" button on and you can see Botton 4 is to view all the running apps. Please see the below picture.
Viewing all running apps: You can click the navigation ball on the screen and tap this icon . Then you can go to the page and view all the running apps on your device.
Customize the frequently used apps:
1. You can click the None button as below.
2. And then click the right button of " Open Apps " to customize your own frequently used app. Then you can click the navigation ball to tap the app icon for quick use. Each Botton on the navigation can be one app. The maximum of navigation is of 9 buttons, the default buttons included.