>pin 4 of the MMC1, which is the A17 control...

Exactly. 28-pin PRG ROMs only have 1 wire to disable the chip, which is pin 20. This is required in order for the size of a 28-pin PRG-ROM to shoot up to 128KB.

CHR ROMs always have 2 chip/output disable lines, since the chip has to decode the A13 line (pattern table enable) and /RD line from the PPU bus. Note that the Konami board #352026 (Super C, Mission: Impossible; equivelant to a TLROM) uses an extra OR gate on it so that a 128KB CHR ROM chip only has to have 28 pins on it.