Thanks.I don't blame you for not wanting to go spending money on a dedicated power supply just for this job. What I'd do is put the spinny HDD into your G4 tower (but don't screw it down into a drive bay: just leave it loose in there on the floor of the box, so you've got wiggle room to fit the IDE - USB adapter onto it) and hook it up to one of the 4-pin power plugs from the G4's PSU.
You will be using the G4 purely as a power supply. You won't need a monitor attached to the G4 (though it's possible you might need a keyboard plugged in just to make the machine feel happy?). Plug the adapter into the HDD, plug the USB end into your iMac, hit the power button on the G4, and your drive should spin up, and should show up as a mounted drive on the iMac. From there it should be plain sailing.
EXTRA NOTE: In order for the G4 to operate safely with proper airflow, you will need to close the side door on it, so I'd remove the blanking plate from one of the G4's PCI slots and feed the adapter's USB cable out through there. Depending on how long the USB cable is (device cables are NEVER long enough these days!), you might need to use a USB extension cable.
Why would I need to install a HDD into the G4, when I want to take the HD out? In essence , I'm looking for a very simple plug it in and download method, as I have never done anything like this before and the inner workings of computers are a bit of a mystery.
Also, I no longer have the keybord and doubt if I have the power plug to actually fire up the G4. At the moment, it's little more than an ornament that has been dormant for 15 years.