Your third post explicitly stated 'The following URL shows what the back of my G4 looks like with the jacks. Which jack would I use?'. In my second reply - Figure 1 of the
Apple link - shows each 'jack' identified.
Note: 'happed' was meant to be 'happened'.
I did omit, what Khama noted - that a USB mic / headset could also be used (plugged directly into any Mac's USB port). The benefit of the iMic is - you can use practically any 'standard' microphone, including headset based microphones, via the 'standard' (non-unique Apple) microphone jack.
Since the iMic has a headphone jack as well - you can use practically any headphone / headset headphone.
Which specific model headset? - that is a personal decision (sound quality of the headphone, voice quality of the microphone, weight of head set, number of headset speakers, which side the microphone is mounted, etc.); and, some models were mentioned.
Here are some Google
text and
images search results.