Defender td5 won't start and run

Hi I am new to this so bare with me.
I'm pulling my hair out with my 110 Defender td5 1999 it will not start and run but will start ran if I unplug injector 4.
So far I have changed.
Wiring loom in the head and have had the ECU checked ( by ecu testing ) I've checked all the wires from the ECU to the head as well for connectivity.
Had a new fuel pump as well.
And have changed injector 4 for a new one and programmed it in.
New CKP sensor as well.
And still won't run with injector 4 connected.
Has anybody else had this problem ? Or anybody knowanything I can try next.
Thanks in advance

Despite the ECU having been checked, it is starting to sound very ECU like. How else would it know to run with injector 4 unplugged? If it was some sort of short in the wiring it would be apparent whether or not the injector was plugged in. Not all ECU faults show up on a standard set of tests, and it is possible for one to behave itself in the workshop and misbehave in the car, in my experience. I'd be looking at finding an ECU that you can swap in to see if it makes a difference. I'm thinking about getting a cheap secondhand one off Ebay for my TD5 just for testing purposes myself.