What cabling to connect my MacBook Pro headphone jack to the TOS input of my Emotiva ERC-4

You’re gonna need more than a passive cable to convert analog out to digital in.

Computer sound is NOT hifi.

You can't be serious - analog out can only go into an ADC, NOT a DAC. And a DAC only outputs to analog patch cords. If the Emotiva has an analog input just use it direct.

Yeah, that’s not gonna work for many reasons.  Unfortunately your Emotiva doesn’t have a USB input, which limits your possibilities.  You’re better off getting a dedicated streamer that outputs SPDIF (RCA, not TOSLINK) that you can then plug into your Emotiva.  Plus, if you’re at all serious about audio you don’t really wanna use a computer as it’s a bad/noisy source for music.  Sorry to be negative, but it is what it is.

If you have a MacBook Pro that was made in 2015 or earlier, then you most likely have a combination analog and optical digital output. The easiest way to check is to go to System Profiler -> Audio -> expand Intel High Definition Audio. If SPDIF digital audio output is listed, then you can use a mini TOSLINK to TOSLINK cable to directly connect your Mac to the Emotiva - link.

Otherwise you will need to use a USB-C to Type B cable - link. You’ll also need what’s known as a digital format (or data) converter. This will convert from a USB data stream coming from your Mac into a SPDIF data stream that is sent through an optical digital output to your Emotiva’s optical digital input. A good choice would be the Topping D10s (which can also function as a traditional DAC) - link.