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

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.

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.

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.