IMEI number for iphone 4

I have an iphone 4 through Verizon, and I recently received the iphone 5 as an upgrade. I am trying to locate the IMEI number on my old phone so I can unlock it. I have a friend interested in buying my old phone but I want to make sure I can unlock it for them. I have tried a few different methods of locating my IMEI, but have been unsuccessful. Any input would be greatly appreciated. Thanks