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The sync speed dropped significantly over night. So did CPU usage (and temperature). As far as I can tell, it's limited by the network as neither CPU or disk are loaded.
Screenshot:
http://too-hot.dk/storage/temp/rpi_btc.png
But 45% done after about 20 hours are not bad either :-)
Heat management has been a major concern with RPI4. Unfortunately it's limited by technology. You only get a certain amount of MIPS per watt at a certain technology node. (btw. I used to work for Intel).
RPI4 manages it pretty well i.e. throttles down the clock when temperature exceeds 80C - it's built for operating at those temperatures (I hope). So you can either accept a slight decrease in performance at high workloads or add some heat dissipation capabilities. Or play around with under-volting/under-clocking if that's your thing.
I just got a small heatsink for now (http://too-hot.dk/storage/temp/rpi_heatsink.png) which helps quite a bit. As you can see, it's at 60C with the moderate load. It still reaches 80C at full load though even with this heatsink (however, that requires more active processes than 'just' bitcoin).
There are a lot of cool cases for the RPI4 comming out, where the case is made of aluminum coupled to the CPU.