I dont see how you would do this personally - the mk4 wings dont reach all the way down the the lower panel so you will be left with a big gap aswell as the bumpers not fitting/looking right due to gaps etc.
The line of the lights between mk3&mk4 is different - also the mk3 bumper is shaped to follow along bottom of lights.
The rear valance is also completely different (mk3 is lower and the mk4 is much higher) also the rear quarters are 'stepped' and dont continue to the bottom of rear valance so there is no way you would get a mk3 bumper on without it looking dodgy.

By all means have a go at it, but looking wrong isnt always original


Mart.