Assuming you have both phones, take them to the store you bought the iPhone 4 at and have them transfer the data (contacts, events, etc.). Alternatively, if you backed up your iPhone 3GS just before buying the new one, try to get access to that computer if it has not been erased. Plugging your new iPhone into that computer should launch iTunes and offer you the option of restoring your data onto the new phone. (I think you'll have to register the new phone onto that computer first.) Once you have the data on your phone, connect the phone daily to your current computer to back up the phone data into iTunes.