First, test the connection between your transceiver and the OS4 using the OS4's local echo service. https://manuals.sharkrf.com/openspot4/en/control-calls.html#local-echo-service
If I understand correctly then your friend is also using transcoding (you wrote he/she uses a D-STAR transceiver to reach a C4FM network). Tell him/her to also test the connection between the transceiver and hotspot.
If local echo works great for both of you, then you should test the connection between the transceiver and the server. You can only test this if the server has an echo service. FCS servers have an echo service in room 99. Select the server which is the closest to your physical location.
Also note that if both of you are using a D-STAR transceiver over a C4FM network, then the voice gets transcoded twice so voice quality will be degraded. Transcoding degrades voice quality, and also a C4FM network uses the AMBE3000 codec, D-STAR uses an older, lower quality AMBE2000 codec, and transcoding from 3000 to 2000 greatly reduces audio quality.