Hi Jeff,
You have a couple of choices:
1. Once you have made your 4:3 project you can change the aspect ration of the entire project by going into the project file data and "hacking" the file as per Tony's link. Always make a copy of the project file before you do this ...just in case.
2. Render the 4:3 project and then start a new project in 16:9 format and just drop the 4:3 video file in it's entirety into the time line and render as 16:9. Depending on the aspect flags you might have to force the aspect ratio of the video by right clicking > set aspect ratio. This way of course means re-rendering with possible loss of quality but is pretty easy to do vis option 1.