Apologies for delayed response.
G4 behavior is the normal behavior of an android phone. As soon as you connect a USB audio device, audio from any app will be directed to it. However this will be through android audio architecture for most apps and there are quality concerns with this architecture so it is advisable to use app that can bypass it i.e. UAPP, Neutron, Onkyo HF Player etc.
For XA, Sony
here advises to go to USB connectivity menu and click on detect device to detect a peripheral device. I would suggest that you download Onkyo hf player which is a free app and try to use it with 16bit 44KHz flac file. Use the same cable you are using with G4. Make sure no other inputs are connected to iDSD.
1. In the first case connect OTG cable to phone then power on iDSD and then connect OTG cable to it. Launch Onkyo player and start playback if the DAC is not detected please follow the step suggested by Sony above.
2. In second case, connect iDSD and phone through OTG and then power on iDSD. Please note that this will draw power from your device to power iDSD so proceed at your own risk. Launch Onkyo player and see if DAC is detected. if not, follow the steps suggested by Sony.
3. If both of the above fail, try connecting a USB flash drive through the same cable to XA and see if USB storage function works well with XA. You may want to try this before option 2 or even option 1.
I have found someone using DFR with this phone
here, So it should work with iDSD as well. However iDSD auto input detection may be at fault here.