The bottom case slides off easily after removing a few T8 Torx screws.
Locate the battery release tab on the underside of the G4.
Slide the battery release tab to the left and lift the battery out of the computer.
Remove the seven T8 Torx screws from the lower case.
Using your thumbs, slide the lower case toward the front of the computer.
The lower case may stick in the front on a tab just above the center of the optical drive. Be careful as you work the case off, or you may break the thin strip of plastic just above the drive.
Lift the lower case off.