D4 stands for 'Droid Dropbox Directory Downloader', and as its name suggests, facilitates downloads of entire folders from the Dropbox app for Android phones. Quite a specific purpose, but the current app only allows files to be added. To use, download the APK to your phone and install. Then, go to the Dropbox app, choose a directory and long press it. Tap "Share To", then choose this app in the menu (currently still called MainActivity). After a short wait, the folder should be in the downloads directory of your phone!