If you don't want any black bars then you are going to have to crop one or both sides. For PAL, both standard and widescreen use 720x576 pixels, but the pixels are non-square and different. With 16:9, the width is 1.7778 times the height. With 4:3, the width is 1.3333 times the height. After cropping you will have 1.3333/1.7778 = 0.75 the number of horizontal pixels defining the 4:3 image, so you will get some horizontal blurring.
I haven't done it but I presume you would use a standard 4:3 project preset and import your wideband 16:9 clip as normal. If the clip appears squashed sideways, then right-click on the clip in the media bin and select "Interpret Footage". Then change the pixel aspect ratio to conform to 16:9. Then use the Motion property to adjust scale and position to taste. If you need to scale beyond 100% to fill the vertical dimension, then set "scale to frame size" to Off.