You say this as if various heat-range plugs ARE available for four-stroke engines...
They are but only by shopping from differant manufactures. From my experiance from hottest at the top to coldest at the bottom.
Thunderbolt 4C Hottest plug
OS F
Fox Miricle Plug
K&B 4C Coldest.
I haven't tried it but there has been a race club out West that has a class where they race four cycle engines with very high nitro fuel. I understand they add extra head gaskets and use two stroke plugs. I do not know how well they idle, but that may not be a factor for their competition.
Also, there are other plugs I have tried but I cannot remember how hot or cold they are. The Miricle plug is only slightly cooler than the OS but there is a fairly wide margin between the others especially the K&B 4C. I picked up and extra 300 RPM or 600 total by switching from 15% to 25% fuel with the K&B 4C over the Fox Miricle Plug. I got 300 RPM from the fuel and the same Fox plug I was using after retuning. I then put in the K&B plug and found I needed to lean it and the RPM was 10,500 over the original 9,900 or so from a TT .91 FS and APC 14-6 prop. It was a big help with the fun fly competition and I won the climb and glide event and the entire event.
They say the Fox Miracle™ is a bit colder, but its lower price is the only reason people buy it.
I buy it because it is a better plug than the OS F for moderate high nitro fuel such as 15%. The OS F is only better with 5% fuel, but that may be different on other brands of engines. I assume it is only lower in price because Fox doesn't chrome plate it, that has nothing to do with its performance.