A smart editing program like Magix Video Editor or Avid Xpress could ease your life up but perhaps they are out of question here. I would recommend you to crop the sections from 16:9 videos that you don't require. Resizing 4:3 for 16:9 or vice versa is not a good idea. I know that will double the time of your video editing but the end result will be so good, you'll be happy you went for the fuss. Once you get around some editing, you'll find better ways to cropping 16:9 for 4:3 videos or vice versa, if you know what I mean

.