All G4 iMacs had a mini-VGA port for video-out so you don't need the USB solution. Apple no longer sells the needed adaptor but they are around:
That one outputs to a regular VGA port. You may be able to find one that outputs DVI or HDMI for newer monitors.
The problem with USB solutions on iMacs is that early iMac G4s had USB 1.1 ports that may not be fast enough. Later ones had USB 2.0.
To see which USB you have, do "About this Mac" from your Apple menu and, in the resulting window, click "More Info..." to launch System Profiler. Depending on your OS version (which we don't know--consider posting that, too), the resulting display will look like this:
The second line, regardless of the line's lable, will have a code; in the example for a laptop it is "MacBookPro3,1." Yours will start with "PowerMac" followed by numbers. Post just that (no serial numbers, please) and your sceen size (measured diagnonally) and we'll have an exact handle on your specific iMac variant. That will tell us if you have slow or fast USB.
Just a note, the iMac G4s could only mirror their display. They could not extend their desktop with built-in software. This third party software supports extending displays:
Being VGA your resolutions will likely be limited to 1280 x 1024 at best, and 1024 x 768 at worst. Software such as DisplayConfigX or SwitchResX can and more viewing room on external displays.