The PowerMac G4, depending on which modell it is, comes either with one VGA and one DVI connector (the older ones, or with a retail card), or with one ADC and one DVI connector (the later ones), with a DVI-to-VGA adapter being included.
In the case of DVI+VGA, you only need a DVI-to-VGA adapter if you want to connect two monitors, and none of them has DVI.
In the case of ADC+VGA, it's the same, yet you alawys will need 2 adaptors if both screens are VGA, one being included with the PowerMac. If both screens are DVI, you only need an ADC-to-DVI adaptor.