how do i transfer information from iphone 3g to iphone 4

Old Phone to New Phone

http://support.apple.com/kb/HT2109

Back up the old phone and then restore the new phone from that backup. Note, any data you had on the new phone will be replace with data contained in the backup you are restoring from

See the instructions here and here. If the old device contains content no longer available in the iTunes Store or not free to redownload, it needs to be transferred to a computer and then synced to the new device.

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If you back up all your iPhone content to your iTunes acount (sync it) and then connect your new iPhone 4 to your iTunes and sync it again then all the content will now be on your iPhone 4.