A lot of people seem to be saying that the Pi 4 runs too hot, and needs cooling. I thought of passive cooling, and started sawing up an old heat sink into small Pi-sized pieces, but that turned out to be more work than I'd thought. You can buy a Pi 4 set of four heat sinks on eBay for 99p. It's just aluminium with fins.
Then I thought, wait. I have innumerable small fans. I pulled out a 60 cm wide by 25 cm deep fan, because that seemed to be about the right size, blue-tacked it to the Pi (I didn't have any bolts that were long enough and thin enough), then realised it needs 12 volts, and the Pi can only supply 3.3 or 5.
Not a problem. The way I power Pis, is by taking wires from the 12 volt power supply of a PC (usually a PC nearby), and running that over a power-over-ethernet cable to wherever the Pi is. Then I feed the 12 volts (more like 10 or 11 by the time it's run over all that wire) and use a voltage stepper-downer (eBay, £1) to take it to 5.5 volts, which I feed into the Pi.
So I took that 12 volts feed, and fed it onto the fan. The fan is almost silent (I can hear it if I get really close to it and listen really carefully) and brought the Pi temperature down from 56C to 39C. Which means that I can now try to overclock it!