4:3 will usually just be a centre punch of the 16:9, cutting off the sides and any text outside as well.
The only way to be absolutely sure it won’t happen is to move all the text inside a 4:3 title safe within the 16:9 version. I hate doing it too and wish that 4:3 broadcasting would just go away, but it won’t so I comply. Digital Heaven have a simple 4:3 title and action safe generator for 16:9 projects. Just drop the clip on top of your text and it will show your safe areas.