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