It automatically does /4 based on FSB. If your FSB is at 133 then the PCI bus will be 33MHZ. If you up the FSB, the PCI bus gets upped.
Rumor has it that the 7-5 BIOS (which is not on the US site yet) will allow you to modify the divider but the only person I've heard that tried it ended up with a paperweight. (Have no clue if it was operator error or bad BIOS or just a flash gone bad.)