I was personally curious as to what's different with XFX's ATI BIOS'es compared to the others, but I could not find anywhere to download a copy of the BIOS. So, I decided to look on XFX's own site, and download their "BIOS Update" for the HD 4870.
Here is where I got it from:
So I downloaded the "Windows" version of the updater.
Then I found that it did not have a BIOS file in the archive!
So I looked further with a Resource Editor. I found that the main flasher EXE file had a BIOS image embedded inside. But, to my surprise, it's NOT for RV770 (HD 4870)!
So then I found that several people are saying the BIOS update didn't work. This may be why.
In case anyone's curious, I've made a dump of the embedded BIOS file and uploaded it here: