Unfortunately, the G4 does not have enough room, internally, to mount two optical drives at once so you will have to remove the old one.
This is actually quite easy,
Shut the machine down, remove the power cord and open the side door.
You will be able to see where the current optical drive sits by following the flat ribbon cable up to the front of the machine. The Hard Drives live in the bottom of the case so there is no chance of mistaking the location.
The optical drives on the G4 come out through the front of the machine. Look at the front of the machine and note that the part of the front that drops down, when the CD is ejected, is attached to the part below it. This whole panel is removeable. With the door lowered, take a look at the area around the right hand side of the CD slot, on the inside of the machine. You will see two plastic tongues which are the same color as the front of the machine. Push both tongues, gently, toward the left hand side of the machine. This will release the front cover. Peel it off the machine by swinging the whole panel to the left. It will come off the front of the machine. It is held, on the lefthand side, by two plastic tabs. When you put the panel back, these go in first and the panel is swung, as though it is hinged, back into place.
Now that the panel is removed you will see that there are two screws holding the CD carrier in the machine. Remove the ribbon cable from the back of the CD, remove the power cable and the audio cable. Remove the two screws from the front of the carrier and slide the whole thing out. I just know you can figure out how to remove the four screws holding the CD onto the carrier.
Installation is the reverse of removal. The ribbon cable will only go in one way, as will the power and audio cables.

Hope this helps

Good Luck

P