Zed already has coilovers (well, at the front anyway!)
If it's only one-way height adjustment you're after then just do the springs, as it's a far more economical way of achieving the goal. If you want to improve/alter the ride characteristics in a more dramatic fashion then you need to start thinking abut changing both dampers and springs, but you really need to look at damper and spring rates in detail to keep the car drivable for you personally. A set of aftermarket one way dampers with the correct springs will give you the drop you're looking for but will also firm up the ride and you'll have no way of altering it if you don't like it, whereas if you went for a set of fully adjustable for both bump and rebound then you can adjust to your hearts content (within the parameters of the damper itself, of course) but you'll pay well for the privilege. Of course, if the non-adjustable ones give you the feel you want then that's the set you buy!
I'm not sure what the stock damping/spring rates are like on the GT4, but I'd start by finding those out and then getting the details of other options from the manufacturers and seeing just what is available out there in terms of difference. It'd be very easy just to say "Oh, go and get this set because I've got them and they're ace!" but without knowing specifically how you want the car to feel then it could ruin the car for you and be £800+ down the drain.