On my previous Z I had Eibach Pro-Spacers fitted, 15mm front & 20mm rear, however in hindsight I could have got away with 20mm front & 25mm rear, they're an absolute dooddle to fit, they simply bolt to the hub using the bolts supplied & then the wheel bolts to the spacer using the original wheel bolts. However I do believe Eibach have two different kits which fit differently, the other version using longer wheel bolts which go straight through the spacer to the hub, so make sure you order the right ones! Obviously the sizes I had & will have again once funds allow may not be suitable for your car with the GT4's, whilst looking down from above the wheel arch, just measure from the uppermost point of each tyre to the outermost edge of the wheel arch, this will give you a maximum spacer size whilst still abiding by the law, if you don't want the wheels at the extreme edges of the arches then you can of course order slightly smaller spacers, whatever takes your fancy!