I just thought I would share this in case it helped someone else.
I have my rPi4 running a media centre mounted behind the TV, and yep, that is one very noisy and annoying fan.
I simply bought a 12V fan online (of equivalent dimensions) and replaced the 5V fan on the board. A 12V fan running at 5V is much slower (and much, much quieter). I would have preferred something in-between 12V and 5V, but couldn't find one with the same dimensions. The connectors were not compatible so I had to improvise there a little, but I made it work. I wasn't sure if it would be too slow to be effective, but it appears to work pretty well. I have been running it this way now continuously for a few months.
So now the fan runs pretty much all the time, I cannot hear it at all, and any time I check the processor temperature, it is around 50-55 degrees C.