If you choose 16x9, some phones still takes a 4x3 but just automatically chops an equal amount off the top and bottom to make it 16x9. So it saves some space and its an aspect ratio that's preferred for some uses.
On those phones, if you want 16x9 output all the time and rarely mis-frame (vertically) pick 16x9. If you want to use some as 4x3 or maybe wish you had aimed a bit higher or lower for a 16x9, go with 4x3 but you will have to crop any you want to end up as 16x9. So its an extra step.