If you are performing a video capture, CTRL + drag will lock your selection to 4:3, Shift + drag will lock your selection to 16:9. For image capture the square lock is the only ratio we lock, unfortunately 4:3 is not present for image capturing.
We do appreciate this feedback, thank you for taking the time to share.
Is the 4:3 image capture now available in verison 12 of snagit? In my mind, whatever dimensions you key into the snagit capture, you should be able to proportionally resize that box and maintain the ratio be a ALT+resize or something.... A lot of people would like this feature but have yet to articulate their desire of it just yet! thanks.