With the 4/4 it is like this. The ECU is in the fuse box in the direction of travel on the right in the engine compartment directly behind the windshield. This fuse box is made of aluminum sheets and the cover is attached with three thumbscrews that you unscrew first. On top of the ECU is a smaller box that contains the immobilizer. Do not confuse! The ECU is the larger box that lies at the bottom.
On the 4/4, there is a large plug connected to the ECU, the size of half a cell phone. You can't just pull this plug off. It is firmly locked with an Allen screw. I don't remember if there is a plastic cover over the allen screw that you have to pull off first.
Before you do anything, the battery must be disconnected.
I don't know how much the whole procedure differs on the Plus 4 compared to the 4/4.
