The wiring looms fitted to these cars are made up of a common main loom to all vehicles with accessory or sub-looms for additional equipment that plug into the main loom. The main loom comes in different flavours depending on the engine fitted. Fuel injected cars have a different main loom to carb fed cars as the main loom includes all engine wiring. Also, cars fitted with an OHC engine have a different main loom to those fitted with a CVH engine. The equivelent MK3 and MK4 looms are very similar in most respects as most circuits are identical and follow the same routing in both cars. However, the higher spec MK3 cars have a fancy 'fault diagnostic' computer interfaced with the main loom that takes feeds from allsorts of sources and displays it's information by warning lights on the dashboard. This means the MK3 loom has extra wiring, it has extra sensors such as float switches in the cooling system expansion tank and screen wash tank and has a level meter built into the dip stick for low oil content warning. This is all well and good but the cars are known for suffering problems when this kit gets a bit long in the tooth and end up giving an impressive christmass tree light display on the dash whilst refusing to do anything such as let the engine start.
The MK4 cars loom is a much more basic affair with none of these extra sensors and so although it's not as clever, it also has none of the associated problems. The multiplugs for the switch gear is also different on the MK4 loom which uses different switches so if your planning on changing your dash to a MK4 dash, then it really is advisable to change to a MK4 loom aswell. Especially as you won't have any of those fancy lights anymore for the computer to light up. Upgradeing the rest of the cars electrics with powered mirrors and central locking etc. is also going to be a lot easier with the MK4 loom in place as some of these systems changed between the two MK's and the MK4 stuff is a lot more readily available.The extra equipment not fitted to base model cars such as electric windows use a sub-loom for each system that plugs into the main loom. I do not know for certain weather sub-looms for MK4 equipment are compatible with the MK3 loom.