SAMC21 - SDADC - VREF B - which pin?
I just checked the SAM C 20/21 datasheet on microchip's product page for the ATSAMC21.

Maybe you have a different version of the datasheet, because in table 6-2 on page 30, it shows SDADC/VREFB on PA04 for the C21 E/G/J - peripheral function B. It's possible they forgot to include it, and then added it later after you had looked.