is it feasible to add coaxial / optical outputs to Raspberry Pi 4 ?
If yes , what is the additional cost?
The HDMI outputs can give audio but many Digital to analogue converters are pre-amplifiers/ amplifiers do not have HDMI inputs.
If somebody can help me in resolving the issue.
There are a lot of options for adding audio outputs to Raspberry Pi’s. Search Core-Electronics for “Raspberry Pi Audio”. The Pi version 4 is very new and the layout is different for the USB ports. Some of the boards have connectors to the USB ports which won’t work on the v4 Pi’s. It’s also possible their could be issues with the software to drive them if the manufacturer hasn’t checked it on the new Pi v4.
At this stage I would either wait a few months for the manufacturers to catch up or buy the Raspberry Pi 3b+ with an audio board that suites your purpose.