(adsbygoogle = window.adsbygoogle || []).push({}); I am planning on selling my iPhone 4 but most of my contacts are on the iPhone and not on the sim card. How do I transfer all of my contacts from my iPhone to my sim card?

I think you can't do it on a new iPhone. You have to put them on your itunes. But if your iPhone allows installing third party software (when you have done a so called 'Jailbreak', you may download a piece of software called iSIM. This will allow you to batch copy all your phone numbers to the iPhone without the need for a PC or Mac.
What's more, some related toturials about iphone Contact to computer.
For Mac:You must login or register to view this content.
For PC: You must login or register to view this content.
Hopefully these work for all of you like it just worked for me!

You can do it using iTunes, but it's giltchy I've lost all my contacts a few times ( did I ever mention I ****ing HATE iTunes ?)
There are no option in the iPhone to copy your phone contacts to the SIM, you can only do the opposite ( Apple.. )

you would need to jailbreak to get the iSIM app from Cydia (i dont know if that's the real name) has not been updated to run on iOS4 (well last time I checked) maybe it had now