Yep, 1 + 2 = 4 leaking back excess, if you just replaced 1 + 2 you'll find that 4 will leak back more. This is what we could never get through to Ford when they were in warranty.

We had to spend ages doing a load of tests ( leak off 1 min 4 min 8 min , compression test, fuel sample , live fuel pressure tests ), and phone them with all results for 'prior approval' to replace the injectors, they would only allow injectors in the cylinders that failed their tests - knowing full well that once the 'failed' injectors had been replaced, those that were close to failing would be out of spec.