There is a pi 4 specific raspian for the new board. I suspect the old version will not work since there are extensive hardware changes including the change fro A53 to A72 CPU
The Pi3b ran with around half an amp (but could draw more if it is running other devices). Does this Pi really draw about 3 amps or more? Or is that just what it needs for the devices? That could be a big drawback for imaging in the field.
they won’t draw power with nothing plugged into them...that is t how it works, but I use a 12v to 5v 4 amp converter so no worries here, mind you I have monitored my rpi3b+ with all ports used and it’s never drawn more than 1.5 amps, but I have all power Hungary devices plugged into a powered hub, namely my Pegasus UPB..
With my SX USB 2.0 CCD Cameras I have no use for USB 3.0, ATM anyway..