I have four different HTML pages in Dreamweaver, and they all have been made from one file. I have a sidebar in each page, and I need to keep it updated over all of the pages.

What I mean is, on my home page, about page, contact page, and downloads page, I have a sidebar, and the information must be the same across each one of them. But, I don't want to have to edit the same thing four times, so.... How can I link all of the sidebars, so that when I edit one on one page, all four of them change?