Why pin 4 for CS in Zero Audio example?

Community Suggestions for the Arduino Project
1

In this example:

It suggests using pin 4 for the SPI chip select.

But the hardware chip select pin for the SPI header is pin 16/A2 on the Zero.

Perhaps this doesn't matter; but if it doesn't, I'm not sure why there's a hardware pin at all, and this page states that even if you use another pin, the hardware pin must be left set as output:

Of course this info might be out of date and only apply to Atmega chips; I'm not sure. But if it is accurate then the logical thing to do would be to specify pin 16 be used in the tutorial.

2

A really sane question with (again) no answer...

3

I'm not sure why there's a hardware pin at all

The hardware pin defines that the Arduino is an SPI master, as opposed to an SPI slave. You could use that pin as the slave select pin for the SD reader, too.