from what i've read so far i think it's safe to say EFI is a replacement of the current bios software and some intel boards already support it. Which means that some of us might be able to run 10.4.4 with a bios update.
Provided of course our beloved maxxuss (or any other RE expert for that matter) removes any protection systems.
Then again apple might have developed there own version of the EFI which is not entirely compatible with what's on the net now. In that case someone might have to modify a current EFI, copy/hack apple's EFI or whatever.
or am i totally missing the picture?
to answer your questions:
1 No, a new type of BIOS
2 not a clue
3 it seems the EFI can/will provide a sandbox in which the os runs. managing network and other i/o. So: Big part
4 That of course depends on your system On some intel boards it's possible. But i can't say wether it would be compatible with apple's EFI.