As I said before, attachd to the GR4 force is the whole fast-jet nav training system from recruitment, selection, Domine, Tucano and Hawk training.
The saving would be gained across a far greater spectrum of the RAF than would be gained from simply disbanding the GR9s.
There would ne no need for sudden death on the GR4 either. Simply turn off the training tap.
AFAIK recruitment was frozen anyway. Stop first the Dominies as the last courses go through, with maybe a smaller sqn for the ME nav stream. If the MRA4 also goes then all the Doms could go. Then the Tucano sqn as the last guys progress and then the Hawk. The Reds could get what is left if the Nav Hawks and that would prolong the life of the Reds with no additional procurement cost.
For navs in the training system I could foresee really hardball. No recourses simply to get them up to speed - hack it or go would be the order of the day.