because the topic has "Adobe" in it i am guessing this has something to do with photoshop? If you are using CS there is a preset for 16:9, you could just create a blank image and past your 4:3 into it - be sure to maintain aspect ratio. Than zoom in and re-frame.

You can also check out a pretty good piece about Widescreen production here: ETV Cookbook

If you don't want to read the entire thing here is the part about photoshop:
If the graphics are instead created in a computer tool such as Photoshop, many designers are used to sizing the graphics for import into non-linear-editors using the non-square pixel structure of digital video (720H x 486V). Often, 4:3 graphics are created using a canvas of 720H x 540V, and then resized before export to 720x486. The result when imported into the NLE will be correctly scaled images. For Widescreen production, the designer should instead work with a canvas of 864x486, and then scale to 720x486 upon export. (NLE systems using 480 vertical pixels should begin with 853x480)

There is also a good piece at the Editors Guild