will be looking to sell my iphone 4 to get the iphone 5 when its released.
its locked to Vodafone and because i dont have the best signal at home it does drop the odd call.

when is the best time to sell the iphone will it be worth more if i sell it before the iphone comes out and use the sim in another for a while

Yeah you would get more for it now than in September.

what about July say soon as it is given a date for release

Doubt it will be given a date in July, the rumour is that it will get announced in September with the iPods.