The ”ISO B4” paper size is 250 mm × 353 mm and the Japanese variant ”B4 JIS” paper size is 257 mm × 364 mm.
Dorico setup uses the ISO B4 and the print dialog seems to use the B4 JIS variant. I don’t know if this is due to my printer, but it would be helpful to have them named as ISO B4 and JIS B4 to avoid confusion. In my experience almost all the printers use B4 JIS and don’t easily print ISO B4.
Yes, the list comes from the printer. Printers usually have the JIS variant - probably because Canon, Ricoh, Brother, Kyocera, etc are all Japanese companies. My printers (HP) show “JIS B4” in Dorico’s drop-down list. They also have “Envelope B5”, which is ISO, but not the ISO B4.
If you’re on a Mac, it is possible to edit the PPD (PostScript Printer Description) file that the printer driver uses to get the list of paper sizes, though.