I've done this - and its not easy. The Mk3 used most directly switched stalk circuits whereas most of the Mk4 ones are relay switched. You also have problems with the indicators as the Mk4 fuse box has no provision for the flasher relay, and with the horn IIRC as the Mk3 uses a switch on the +ve side and the Mk4 a -ve.

Its not just a matter of matching up the colours. If I get time I'll write down exactly which Mk3 colours match to which Mk4 colours. I did get it all working in the end including the variable wiper delay and the auxilliary warning lights.