Well I've been mapping the pins today and I found out what happened to the fourth UARTUSART. It seems that it's alive and well and available on pins 65 and 52.
Where the heck is pin 65 I hear you ask? Looks like it's A11, I'll post the latest pinout diagram soon.
So the Due has 5 hardware serial ports, 1 for the USB and 4 for the user.
However it doesn't appear to be officially supported yet.
Rob