No, DH is an app on its own. To download and flash the FW is free and you don't need to pay. Please note that the 4G Transmission will not work if you are using any FCC hack. Download the app from below link.
https://drone-hacks.com/
Do you need to use the modded DT DJI FLY app in the USA to use the Mavic 4 Pro with the Enhanced transmission?
I'm going to guess not because it's already in FCC here but I don't know for sure yet
He would have got it from Dank Downloader. This is the only other place that offers all Dji drone FW.
@emirkurtpro I would not recommend installing that FW. Its beta FW and will void your drone if it crashes