The big question is why do you need an external app for flight planning?
I have Garmin Pilot and ForeFlight EFB when flying in my C152/C172/P-3/EC120B in real life. Thats why the EFB can transfer flightplans to the GNS530 or G1000 in real life too. To make them aligned. Same should be with the AFS4. The flight plans have to be aligned between sim and the EFB in order to make proper use. Otherwise the sim is not usable as a training device, which it is supposed to be for me.