Firstly, I am not an Apple user.....
But looking up Darkroom, it does not support raw files in & of itself but relies on the Apple OS to natively support 'reading' camera raw files, saying Apple have regular updates and if your camera is not yet supported it will be soon.....!
IMO your next port of call is to search about your iPad OS version and whether it currently says the Sony A7R4A raw files are supported and if not when it will likely be added by Apple?