I don't think the SDIO 4-bit mode can be implemented on Due. It appears that I/O line PA21 is used to drive the TX LED.
The High Speed MultiMedia Card Interface uses PA21 for MCDA0, data line 0 of the SD.
I plan to play with 4-bit SDIO mode sometime in the future for SdFat.
There are drivers for STM32F4 and I have an Olimex STM32-H407 so that may be first. This is an amazing card for about $39 STM32-H407 - Open Source Hardware Board.
168 MHz, 1 MB Flash, 196 KB RAM