keep your mk4, dont think mk3 fiestas are as good as all these fiesta owners make out, yea they are like a small go kart, but you can wack coil overs on a mk4 cabby and thatll be like a train on rails too, the 1.8 zetec in a fiesta isnt powerfull atall in weight comparison imo and tbh at i guess id say the mk4 cabby wouldnt weigh a hell of alot more unless the fiesta has lost weight through its usual rust issues, if your mk4 cab has all been done by you as said above you could be wasting your time and be opening another tin of worms getting the fiesta, id keep your mk4 if i was you