As far as I know, there are two other ways of powering a rpi4. One of those is with a PoE Hat for the rpi4 (Buy a PoE HAT – Raspberry Pi), or powering it through the gpio pins (see diagram below).
Both pins 2 and 4 should be connected to 5 volts, you can use any one of the pins, but it is advisable to use both pins as if one pin is unable to provide accurate 5 volts the other pin can compensate for that.
Can't answer for the PoE hat, but I believe the power from the GPIO pins doesn't go through all the power protection stuff on the Pi, so be careful not to accidentally get it wrong while wiring & avoid cheap power supplies more likely to surge.
Thanks for those replies. I tried powering it using a regulated bench top power supply set to 5v. I used one of the 5v GPIO pins and a GND down at the far end of the header. The RPI powered up and seems to work well, although it is reporting low voltage in Octoprint. I was only using alligator clips to carry the power, so I'll try something a bit more permanent now.