Craig,
Hope you are not reading this post after your test,not sure of the time difference between Australia and USA.
As Jon and Jim have already stated it is very hard to try passing on all the info you require but a lot of it will depend on your natural ability.Some people can pick up a handpiece and show ability straight away, some people can become proficient welders with appropriate training, and some people unfortunately will never become proficient welders.
The 4G (overhead) weld test is not a particularly hard weld test, what is hard is the actual position.(Does that make sense)What I am meaning is the fact that all the hot weld spatter is falling directly onto your helmet,down your neck,down the sleeves of your welding jacket and generally into every little hole in your protective clothing.
1 Make sure you have a good leather welding jacket and welding hood.
2 Make sure you are welding at a comfortable height.
3 Make sure you have clean welding lenses and correct shade lens.
4 Keep your arc as short as possible (as your rod burns away you have to keep reminding yourself to keep pushing upwards)
5 Use a cutting disc to make a straight groove (only lightly) This will give you a straight line to follow across the plate.
6 Don't try weaving your weld, run lots of little hot stringers.
Other than that, Best of luck,
Shane