Hi I am about to jailbreak my iPhone 4 but before I do that, I would like to back up my contacts to my computer. How is that possible?

I used copytrans contacts to back up my old iphone contacts to my computer and then transfer them to my new iphone 5.

in your case, just back the contacts up and then import them to the same iphone after the jailbreak. When you export, make sure to select the "iPhone, iPad and iPod Touch" option.

Thank you! I saved my iphone address book on my computer as per your instructions.

You can use outlook express to back up contacts and messages.
I have done this but it was not the easiest thing to do.
You need to go to info in your iPhone settings in iTunes, select where you want to back it up to in outlook or yahoo, windows or google, then sync your phone and job done.
What's more,these articles maybe can make a effort of transferring iPhone contacts to computer.
* How to transfer contacts from iPhone to PC
* How to copy iPhone contacts to Mac
Hope it worked for me as well.