So, here is how I do it:

- get the Mac 4870 rom available in this thread (named 4870.ROM)
- get the 128k vgabios of your PC card (named pc4870.rom)
- get tyhe fixrom.py script that I attached here

run the following commands in a shell (either MacOS X or Linux)
Code:
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dd if=4870.ROM of=efi.part bs=1 skip=63488 count=49152
blocknum=`printf %d "'\`dd if=pc4870.rom bs=1 skip=2 count=1 2>/dev/null\`"`
size=$(($blocknum * 512))
dd if=efi.part of=pc4870.rom bs=$size seek=1 conv=notrunc
python fixrom.py pc4870.rom pc4870.rom
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first one extracts the efi part of the Mac rom
then get the number of 512 bytes blocks of the bios part of your pc rom
then converts it to a size in bytes
then injects the efi part right after the bios part
then the script fixes last rom indicators and checksums

You can then flash the pc4870.rom to your card, using FreeDOS for instance

If you want to do manual modifications to either part of the rom, don't forget to run the fixrom.py script after.

This should work with most (if not all) of the 4870 cards. If it doesn't work for you, just pm me a link to your pc vgabios.