Thanks for your FAST response!
The projector and screen are both 4:3.
When you say ‘dump the timeline in a 4×3 sequence’, do you mean that I should create another project in 4:3, then re-open the 16×9 project, copy everything in the timeline, then open the new 4:3 project and paste it all into the new timeline, and then export normally?
Thanks much!