I sure hope when you did the Jumping that you Never accidentally applied the 12v Black/White wire power to the Brown/Black wire as this Will destroy your ECM?
The ECM supplies the Ground side for the FCR coil on the Brown/Black wire to energize it and run the fuel pump.
- Do you hear the FCR click at Switch On for a few seconds?
- If you are only seeing 4v on the Brown wire for the Fuel Prime duration, you may have high resistance contacts within the Relay itself, a faulty relay or poor contact connections within the relay socket. Are there any visible signs of corrosion in the socket?
- Try swapping the FCR with your Hi Beam Relay. The four relays being Hi Beam, Lo Beam, ESR and FCR are all the same type.
Good Luck. Let's know how you get on.