does anyone know how to put a Moto-X SIM card into a G4 phone?
If you mean SIM card from a Moto X1 or X2 on the 1.0 or 2.0 plan, those SIM cards cannot be used in your G4.
If you bought the G4 from RW then it should have come with a SIM card. It is on a credit card size card. See here
@amitl is correct. I just want to add that the SIM card in the X1 and X2 must stay in that model even if you sell or give the phone to someone else. It does not contain any personal information.