Either the 4th person doesn't actually exist because the office is understaffed, or someone will be going to a nearby office, or ultimately after 2 years they might be offered VR.
The idea is to have a pool of area reserves to deal with sickness spikes, resignations and (yes please) a syndicate lottery win.
The union isn't simply backing this, they were the ones who dreamt it up.
The union for some reason has decided to hatch a plan to make our lives harder and save the business £150 million.
Simply to prove it isn't their business to run (into the ground)
My guess is RM know this will fail. They could make it fail if they wanted to (as we could with any trials)
They are prepared to write off another Christmas to further weaken the union through people leaving the business or the union.
Then in future negotiations about anything at all they will bring up the fact that they listened to the CWU over USO reform and it was a disaster so they won't be listening to them ever again.
We will just watch Walsh and Ward ride off into the sunset (with Ward still muttering about expanding the role of the postal worker while the nurse is getting his jigsaw out.)