Set up a project using a 16:9 template and add all you assets. On the ones that are 4:3 open the pan/crop window and with the cursor on the first frame, right click on the video and select “Match Output Aspect”.
Make sure this is the only keyframe or you will have unintended movement.
A number of years ago I purchased a bunch of extensions from VASST–work great! One such extension is EventTools, which offers a “Match Selected Events to Project Aspect”. What this does for 4X3 to 16X9 image conversion is to eliminate the letterboxing by zooming in to the frame and holding this framing consistently throughout the chosen video clip(s). Then you can use pan/crop if someone’s head is chopped off on individual clips! I use it all of the time and love it! Looks like the extensions still may be available: