If you have an imac G4, then you *do* have a modem on the machine.
The modem rj11 port is right above the rj45 connector you use for
your internet connection (unless you're using wireless) Just plug a
phone line into that port, and you can fax away, as long as you have
the phone number to where you want to send the fax.
The ports are on the back of the imac, on the right hand side (if
you're facing the front of the machine) just above and slightly to
the left of the on/off button. This assumes you're in front of the
machine and checking behind it for the ports. Obviously, reverse
directions if you have it turned around.
But to elaborate on a point made earlier, Yes, you can fax over the
internet, but it generally takes a bit of setup, and some of the
services are not free, so there is a cost involved as well in most
cases.
But, you do have a modem, and can use it for faxing if you so choose.
