Hi Matt,
The only real external difference between the GR1 and GR4 is the FLIR pod on the port side of the forward fuselage next to the LRMTS pod, there's a new cockpit too but don't think anybody has produced an updated cockpit in resin or photo etch. As such all you really need is the FLIR pod, however, in 1/48 nobody produces one on it's own and the only one I can think of is the Flightpath set which contains a white metal example but its a lot of money if that's all your looking for. You do get a lot of other great stuff within this set which you would need to make an accurate GR1 or GR4 but it depends on how accurate you want to go and how much you want to spend.
If I were you I'd either see if anyone has a spare FLIR pod to hand (think the Hobbyboss kit comes with one) or make one yourself.
Model Alliance probably produce the most up to date decals for the Tornado that I can think of, so you'll probably be struggling there without doing your own.
Sorry no easy answer for you!
Murray