To be sure, the GR4 is no doubt getting long in the tooth (it was when I left 31 Sqn is 2001), and I left the Typhoon Flt Test program prior to any of the gnd packages being tested. However, I'm not sure that it's time to get rid of the GR4 just yet and be faced with a similar situation to the one we were in twenty odd years ago but in reverse order - back then we were faced with a gap in fighter coverage, so they turned a bomber (the GR1) into a fighter (the ADV) with mixed results. Now we're turning an air superiority fighter into a swing role aircraft - to early yet I suspect to pole the jury on a verdict. Just as a for instance - can the Typhoon carry a Storm Shadow sized weapon, or a GBU-24 type store - Tornado can, Harrier as far as I know can't.
Good question though!