Can confirm that flashing this firmware onto a 4 port card with the Sil3114 chip, However you will only be able to boot from the two ports on the rear of the card.
The model of the card is a generic SATA3114-AO1.

I wasnt able to boot from the two ports on top, however they may still work from within OS X for storage (Dont have the gear to try it out just yet)

I didnt have any luck trying to flash the card with the tool linked on the first page, so i ended up using Flashrom.
Here is the link: https://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/repositories/1.2/pkg-html/flashrom.html

I also couldnt be bothered messing around with floppies, so i just created a FreeDOS bootable usb drive with rufus, and then just copied over the firmware.bin file from the first post and also the flashrom utility (make sure these are on the root of the usb disk.
Installed the card into a free pci slot into a pc, and then type "flashrom --programmer satasii -r backup.bin" 
This will backup the current bios on the sata card.
And then type in "flashrom --programmer satasii -w firmware.bin"
With any luck, the flash should be successful!
Turn off the pc and remove the card.

Install the card into the powermac (using a MDD, still need to try it out on other powermacs)
connect your sata disk, and boot into your OS X install DVD, open up disk utility, and with some luck it should display your sata disk! go ahead and install OS X as normal.

Please note that this has only been tested on a G4 MDD 1.25DP with Tiger.
I have also noticed that when you boot the mac up, it will go to a flashing "?" icon, but it still boots, the apple logo will appear after a few seconds. Insure if this is due to my drive or the sata card.