4:3 or 16:9 specify the ratio of X to Y.

Common 4:3 resoltions are 320x240, 640x480, 1024x768 for square pixels (This is why computers have these as options for desktop resolution) and 720x576 for PAL aspect ratio (1.067) pixels.

In order for 720x576 to be widescreen 16:9...

720/16= 45 * 9 = 405

so 720 wide @ 16:9 res is 405 high.

In order to force that to 576 height...

576/405 = 1.422

So you would need a pixel aspect ratio of 1.422 to make it happen.
In other words if a pixel was 1mm high it would have to be 1.422mm wide

Confirmation: 720 / 16 * 9 * 1.422 = ~576

However, no display devices in existance have a Pixel Aspect Ratio of 1.422, so anything you displayed it on would have to interpolate the image which would result in quality loss.

Not sure how the hell you rendered 16:9 to 720x576... Maybe it resized the frames? Does everything look tall and thin?