Hi
Cropping to 4:3 in effect, trims your image down. by cutting off the sides. This leaves those particular clips with blank areas to either side that will appear as black bars in a 16:9 preview screen. You can change the preview screen to 4:3 when they will not appear, however, 16:9 clips will be made to fit and will then show bars at the top and bottom If you export at a 16:9 AR, a 4:3 clip will naturally not fill the frame hence the black bars to each side. Mixing AR in a project will produce bars somewhere...so it's best to keep them all the same.
If you crop all the clips to 4:3 or zoom 16:9 clips to 4:3 or set all the clips to show 4:3 areas (if you want 4:3) then you should have no bars in the preview if you set THAT to 4:3. If you export to a 4:3 output then you won't have black bars....but most viewers are 16:9 AR therefore you will have apparent black areas to either side of your project. It's a bit like projecting a 2 x 2 slide on a rectangular screen.
The latest VP version (7.11) has an export option which will letterbox or crop and zoom the project if there are mixed AR. The latest Beta version(7.12) still being tested, allows you to set the AR of the preview to different options but it still requires some work.
Nat