then you need to weld a mk3 top engine mount to the mk4 chassis leg and sort the wiring out, although that bit isn't too hard, the g40 engine loom basically needs power and earth and it'll go. Leave the mk4 loom in for all the ancillaries such as headlights etc...
Fuelling is also an issue, you'll need to do some research, I believe the fuel pumps in the later multi-point-injection mk4 polos flows a similar amount to the g40 pump but don't just assume this is right! The safest option is to keep the mk4 in-tank pump as a lift pump and then also use a normal g40 in-line pump, this is the safest option.