I'm in Australia and it seems that people talk about there being a difference in the p4 here and the ones sold in the USA, if so could anybody tell me what they are.
You'll find that people talk about all kinds of things - and not all are true.
Apart from the power cords on the charger, there's no difference between the Phantoms sold in the US and Australia.
Sounds silly but does the p4 have a built in mic? I've seen lots of videos some with lots of wind noise others without. There seems to be a lot of different distance that people get, I think it quotes about 4.5 kilometres what can I truly expect?
Yes... it would be silly to have a microphone on the Phantom.
All you'd hear is the swarm of killer bees that gets boring and irritating after about 3 seconds.
You've seen old videos from gopro cameras mounted on drones - no-one else records sound.
There seems to be a lot of different distance that people get, I think it quotes about 4.5 kilometres what can I truly expect?.
And last of all would the p4 be the best unit for somebody like myself to buy to who has never flown anything,
Range is variable and affected by a number of factors.
Your local environment is probably the most important.
You will get great range in wilderness areas or over the ocean and much less in urban areas.
The P3 and P4 are both outstanding machines and much easier to fly than you imagine.
There is however quite a lot to learn to be proficient.
Don't discount the P3 Advanced and Pro as they share many features of the P4.
If you don't need the new features of the P4, the Lightbridge equipped P3s give you most of the performance of the P4 and enough money to buy plenty of batteries etc.