What app do I need to install before using DJI Mini 4 Pro?

DJI Mini 4 Pro can only be used with the DJI Fly app. You can download the latest version of DJI Fly from the DJI Mini 4 Pro Downloads page on the official DJI website. If you use DJI RC 2 as the remote controller, it comes with the pre-installed DJI Fly app.