Converting 16:9 video to 4:3

Hello all

I'm creating a (widescreen 16:9) video for YouTube, Facebook, etc, but want a another video (Letterbox 4:3) video for Instagram. Figured it's better to have a close up video than a widescreen video when people are scrolling through the feed.

How can I convert my 16:9 video to a 4:3 video without creating & cropping everything from scratch? or doing as minimal work as possible with the program doing most of the converting? 
Pictures I don't mind resizing but the captions I have on the video as well needs to change as well. (again, don't mind resizing).