While a world sim is a great backup and very useful until you can get hold of a local sim I strongly recommend getting a local sim when ever you are in a country for more than a week or so. It makes it so much easier to book accommodation and meet up with locals (people are not that keen on dialing an international number to get in contact with you), you can always put the world sim back in now and again to check for messages from back home.
Most normal sized sims can be cut down to fit in an iPhone 4, in fact the one in my iPhone 4 is cut down at the moment.
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