I have been doing some research and trying to find out exactly what heat dissipation is needed on the G4.

The heatsink size required appears to be that of a northbridge cooler.

4.5cm wide and 5cm long is the size of the current heatsink, the height cannot really exceed 5cm either as it will foul the RAM if it is. Also weight is a concern as if it is too heavy it may fall off.

So far the contender looks like this
http://www.zalman.co.kr/english/product/zm-nb47j.htm
These are only AUD$11 retail here.