It should be entirely possible to use OnOff SHIM as it is currently with a Pi 4 and a 2.5A micro-B power supply, providing you don’t have any power-hungry peripherals attached. I’ll solder one I have here up and confirm that it does indeed work, but I can’t see any reason why it wouldn’t…
There’s a chance that we’ll do an updated version with USB-C too, but in the meantime give the current one a shot.