If you buy the iPhone 4 then you can directly insert the sim in it...if the iPhone is locked with a particular network then you need to unlock it to make the mobile accept other networks. To unlock an iPhone the remote unlock service would be a better option because, you need not give your mobile to someone else nor do you have any chance to enter the wrong unlock codes. To get such an unlocking service for your iPhone 4 approach Unlocking4u.com and by submitting the country and network to which the mobile is locked to you will be able to remove the restriction from the respective network. This will be a permanent unlocking solution. Once your iPhone is unlocked you have the privilege to insert sim...you can contact the customer care of directly and order a micro sim or cut down your standard sim to micro sim size by following the youtube videos. If you hesitate to cut the sim yourself you can buy a sim adapter from any shop. The rate differs accordingly whilst it will cheaper in eBay.