So what do we need a third FJ type for, if it can't do anything unique?
A few operationally-relevant things GR4 can do that our other FJs can't...
Carry 2 Paveway IVs, 3 laser-guided missiles, Litening III pod with built-in recce capability, 27mm cannon, advanced IRCM, and enough fuel to stick around for a couple of hours. Plus this is all on one airframe so there's no need to go round as a pair to cover all the weaponeering bases. And if you don't want the missiles you can have 3 Paveway IVs instead.
Field the best airborne optical ISTAR kit currently available in theatre - yes, RAPTOR has come of age at last - with the cannon and 2 Paveway IVs to boot. Sorry DJRP, you're old news.
Sustain operational detachments to hot sandy places on an indefinite basis (19 years and counting, with at least 2 periods of "double ops"). The Typhoon force are already working hard to fulfil their existing UK and Falklands tasks; their future sqns will give them the capability to mount short-term deployments abroad but not on an enduring basis. And we've just seen how long the Harrier force can manage.
That gives GR4 the edge for me in the short-term cuts battle. And a few more, less relevant at the moment, but still unique...
Storm Shadow: do you honestly think the UK AI force would be asked to use anything else on "Night One"? It's never going to be fitted to Harrier by the way, it fouls the flaps or something.
ALARM: GR4 is
still the only self-SEADing bomber out there, unless they've started strapping bombs onto the 'Growler'
All-weather low level ingress: might be useful again one day... anyone know the latest on Iran's ongoing SA-20 purchase?
Brimstone: no-one else can fire it yet. Although I accept that it's not the most flexible weapon out there.
I think it's reasonable to forecast that Typhoon is safest from the axe. However, in a GR4 vs Harrier slugging match, I think the extensive list above trumps the Harrier's strong points (carrier and short field capability) - as a few posters have already wryly observed, you need carriers to justify a sea-going aircraft!