Not sure that is the actual concern. Your next sentence is the one. Who wants your new performance king to get throttled because of overheating?
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If you need the ultimate performance all the time then you are correct and it may be worth the effort and expense of adding a heatsink and/or fan. I use mine to stream and record from an h264 video camera and perform motion detection on the stream, something that a Pi 3 failed miserably at. Provided it does that well I don't care if it takes ten seconds longer to rebuild the software, or perform other occasional cpu intensive tasks.
I would expect that to be the case more often than not, for the top shelf product in the range. Otherwise what is the point in using it?
The "issue" with RPi4 has been that many have bought it under the assumption that it can perform in the same environment as previous models. Overall, I don't believe that to be the case.
There are a number of tests on youtube with those heatsink cases and they seem to perform just as well as fans.
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Run top on your PC (or whatever the Windows equivalent is) I imagine it is probably running at less than 20% CPU most of the time. The vast majority of computers run at low loading most of the time. My pi 4 is running at about 50-60% performing its main role and is running fine without a heatsink.
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All good then, but I think I would still prefer a fan, rather than worrying about throttling.
What about wiring the fan to kick in just before throttling occurs?
So most of the time it's not running, but at times of demand it starts up....
I'm sure that shouldn't be too difficult to arrange.
Indeed, there is a project on the RPi website to do exactly that.
There is "no problem" from my pov. I was just chiming in with my thoughts that, for the most part, a fan is a good idea for the RPi4. But clearly that is a contentious point. 
Yes seen that, but I couldn't use it because it requires access to Pins that are already used by a plug on RF board. I've spoke with Pimoroni, and they have said that considerable mods would be needed to use that product.
I just had a look at the Python code, should be easy to rewrite if the hardware can be modded. Are you actually using the same pins with the RF board?, if not a stackable header would be the solution.
Very easy to drive a gpio for the fan with a simple flow in node-red.
Not sure the hardware side would be quite that easy Colin.
I wouldn't feel comfortable about loading a GPIO pin with a 2-300mA inductive load.
I would probably opto-isolate the GPIO, and drive it from the 5V supply instead (Or use a FET to switch the load).
The mini opto-isolator boards cost is negligeable, and the size of a fingernail, and can sink 500mA at 5V.
The actual flow to control the GPIO would be very easy (Thanks to Bart's vcgencmd node)
There is no throttling with those cases.
I haven't got as far as buying a Pi4 yet, I'm just weighing up options.
Yes, this is the pinout for the shimfan - Fan SHIM at Raspberry Pi GPIO Pinout and the RF board needs access to GPIO pins 1, 6, 7, 8, 9, 10.
It's not worth the hassle, as a lot of physical alterations would need to be made to change the pinmap. It's probably easier to build my own....
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Or wait for the MQTT Shimfan with water cooling 
I'm in two minds about temperature control: Is it better to just have the fan on all the time or put the processor through the stress of getting hot and then being cooled again in a continuous cycle?
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We'll you could always make the fan speed proportional to the temperature.
A slower fan is a quieter fan 
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I didn't mean that the pin could drive the fan directly. A cheap relay board or an isolator board such as you suggest would be required.
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I would just start the fan if necessary. Unless you have a constant CPU load it will idle most of the time anyway.
Not to mention the permanent energy consumption. If you are using a Pi in the first place, you want to conserve energy most of the time. 
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I'm using a FLIRC case on my RPi4 which is fanless and works great, temps are usually less than 50 degrees C
