This video should help you. This does NOT look easy to disassemble. Watch it from start to finish. It looks like the hinge is on the screen. You have to take most of it apart. Try to not break anything. If you need to replace the hinges use the eBay link down below. This includes the brackets.
Video of disassembly (set it to 720p): https://m.youtube.com/watch?v=K-pTfjelLB0
Hinges: http://www.ebay.com/itm/HP-ProBook-4420s-Left-Right-Hinges-and-Brackets-60-Day-Warranty-/331314216278?hash=item4d23ddbd56:g:RvgAAOSwQItUDg6z