I just ordered one from Pimoroni as the PI hut was out of stock of the 4gb version and only had the 1 and 2 gb versions....they were available till I went back on the site then gone, so must be selling fast...
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.