If you have an extra PCI slot open, the easiest way is to install a PCI slot fan like this
Or this one
I have one of the first link in my G4 MDD. The problem I had is it's wiring and plug are located where it makes closing up the G4 a bit tricky and a tight fit.
I listed the second slot cooler as two fans are always better than one and it's still a single PCI slot design.
However, I am not sure if that design will fit properly in the G4 case.
The case design for the slots is upside down. Not sure if there is enough clearance and ability to plug the dual fan in with the G4 case design as is.
The single fan design works, though. Just have to be careful opening and closing the case.
Another option, if you are handy with working inside Macs is to replace the stock cooling fans inside.
I have no experience with this, so I couldn't tell you what faster, quieter fans are available and how to remove and install the new ones.
Hopefully someone more experienced will help you out and add to my recommendations.
The dual 1 GHz QS can run pretty hot.
Replacing the thermal compound is a real good idea, as the old pads are baked to submission (not effective).
Use Arctic Cooling MX-4 to get 3-5ºC lower temp (better cooling performance) as compared to the OEM thermal pads (my test results).
Replacing the CPU fan with a Vantec TF6025 also helps a bit with noise and heat.
Blow the dust out f the PSU as well. The PSU fan moves a bunch of air through the PSU and dust accumulations can get severe.
The Vantec looks interesting but the variable speed thing won't work as a CPU fan where the air is being blown onto the CPU, not off. I think if you were to actually connect the sensor wouldn't there be the risk the fan would then run too slowly to properly cool the CPU since the air temperature being measured would be room air? I'm trying to remember what my stock fan is rated at but I am remembering about 22 cuft/min. The Vantec is rated at 8-24 so if it was running too slowly it would be running far under specifications. I guess if you leave the sensor power cable unplugged then it will just run full speed, according to what some have said.
Japamac, if you have used one of these, how did you deal with cabling? People mention a 3 pin molex that is usually for power but in this case only has one lead for sensor power. I looked at an old CPU fan I have and it has a 2 pin connector, so would the sensor even fit (or is this moot since one may not be using it anyway)? Then there's the issue of the power supply which seems to be 4 pin molex. Did you buy an extension cord for this?
The Vantec is thermally controlled.
So is the OEM.
Both are variable speed, with the OEM maxing at 23.5 CFM.
The Vantec maxes at 24.2 CFM.
Stepping of the Vantec fan is much better than the OEM.
Smoother, and I think faster, sooner.
Trouble with the OEM is the noise at full speed.
Though "advertised" dB ratings are the essentially the same at full speed, I find the Vantec to be far quieter than the OEM.
Before I changed my PSU fan in my DP 1.0 GHz QS, I could hear the din of the OEM CPU fan (full speed) even with the PSU fan roaring away.
Changed both and the machine is civilized.
The Vantec is quieter than OEM.
The Verax fan that I replaced the PSU fan with also helped immensly.
Connecting the Vantec to a molex to a 3 or 4 pin adapter works fine.
So does using a 2 pin approach using the 12v and common to the logic board connector where the OEM connects.
The RPM detector wire is not needed and can be cut off or ignored.
The voltage ratings of the two fans are so close that no adverse effects will occur connected to the logic board.
I try fans with adapter cabling. If I like them, wherever possible, I scavenge and solder OEM connectors to the new fans and use the G4 OEM connections.