Do you have the handbook? should have the code for the alarm on the back, or the valet mode could be set up off the remote.
G4 could be any of a few alarms, if it doesnt have the anti hijack then it may well be a concept 300, your best bet may be a clifford engineer/installer who can reset the valet code for you and service the system at the same time.
What area are you in?
Tony