How do I sync my iphone4 to a different computer so that I can keep all my photos and contacts..?

You'll have to transfer the iTunes Media folder from the computer with which you're curremtly syncing your iPhone. Otherwise iTunes on the new computer will erase everything on your phone and sync it to the empty library.

iTunes: How to move your music to a new computer - http://support.apple.com/kb/HT4527 (click on the section titled "External Drive")

Message was edited by: AjaxOfSalamis