If one were to make a website, should the presumption be that most computers now use 16x9 screens ? or should one site build for 4x3?

At the moment the majority of the screens are still 4x3, but the future is definitely 16x9. Personally I prefer a 980 pixels wide layout (most screens using a 1024x768 size), and a fixed aspect ratio for my websites, otherwise you can't control how it'll look on the visitor's screen.

It also depends on how you want to design your site. If you are looking to have a fixed height page then you can have the wider aspect ratio look regardless of screen size. Quite a few sites fit everything onto a stage like this.

I wouldn't be surprised if 1280x1024 LCDs aren't the most common displays in use nowadays (certainly there are a lot of them out there now), which is a 5:4.