Q. Can you fit a 4sd automatic to a EA 3.9 falcon which is fitted with a 3sd automatic with out any modifications, will the computer automaticly know its now running a 4 sd and compensate or do you have to swap computers . any help will be greatly appreciated

Pretty sure you need to have a separate transmission control unit for the 4 speed. 3 speed were hydraulic so the ECU had no control over the transmission, pretty much a 3.9L Manual ECU. So I'd say you need a separate control unit like the V8s.

also, you may need to modify or make a trans cross member, as the 3 speed ea uses the same mounting position as the 5 speed, where the 4 speed auto uses a different mounting position on the floor pan.(about 2" further forward).
this is not a problem on cars that came out with the 4 speed auto as they have both mounting points on the body, where the series 1, 3speed auto ea only has the one mounting point on the floor pan..