Only difference between B3 & B4 is the mapping of the engine. Both exactly the same 2.0 engine using exactly the same components. I highly doubt if the fuel consumption of the B4 is significantly worse than the B3. All depends on your right foot really. 😜
Most car makers use this gimmick these days with bhp making it look like you’re getting a different engine. If you were allowed to do this on Motability (you’re not, obviously) all you’d have to do is add a tuning box to obtain the higher bhp output & acceleration etc for relative peanuts.