GR4 situation in 2015 is predicated on the assumption Op Herrick will be finished - as happened with the Harrier force, it will become undeployable on a long-term basis with just two squadrons. Post 2020 assumes Typhoon will be able to pick up most of GR4 current tasking - but there's still a lot of capability to add to Typhoon before that happens. A lot of assumptions...