I want to sell my iPhone 4 Verizon to a friend but I don't know how to remove my number from it

Set it up "as new device", copied from http://support.apple.com/kb/HT3828

Summary

In some cases you may want to erase all data on your iPhone, iPad, or iPod touch, and set it up as a new device (for example, after purchasing a new device, you may decide to give the previous device to a family member, friend, or another person for their use).

Follow the steps in this article to erase all content on your device and set it up as if it were a new device.

Products Affected

iPhone, iPad, iPod touch

There are two ways to erase your iPhone, iPad, or iPod touch:

Restore and set up as new

1. Launch iTunes and connect your device.
2. Select the device in the source list.
3. In the Summary tab, click the Restore button.
4. Wait until iTunes has completed the restore process. If you encounter any restore errors, please see Update and restore alert messages on iPhone, iPad, or iPod touch.
5. When iTunes provides the Set Up Your iPhone/iPad/iPod touch screen, choose Set Up As New.

   Setting up your device as new will overwrite the current backup.

Note: If you want to keep the device in factory condition to sell or give to another person, disconnect the device when the restore is complete (when the device displays the "Connect to iTunes" screen). Do not select Set Up As New or Restore from Backup. The new user can name the device and configure syncing options when they connect it to their computer.

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Message was edited by: Ingo2711

You won't be able to do it yourself as it does not use a sim. You will have to do it through Verison, although not sure how they do it.