Need Help dropping voltage on a signal wire
You have a voltage from 860 mV to 5 V and want to map that to 500 mV to 4.5 V.

Let's start by looking at the scale factor.  (4.5 V - 500 mV)/(5.0 V - 860 mV) = .97.  That's almost 1, so almost a pure level shifting problem.

Let's look at the level shift.  At the high end you want to drop it 500 mV and at the low en 360 mV.  That's not too far off from a silicon diode drop.  You might be able to get away with putting a diode in series with the signal.  Anode to Hall device, cathode to the ECM with a pulldown resistor.

It's hard to know what the pulldown resistor value should be since you haven't said anything about the current capabilities of the Hall device nor the impedance of the ECM input.  Start with 10 k&Omega; and adjust from there.