When you change the proportions of the template the app tries to readjust all the elements in it by stretching or shrinking, so some of them may distort. The only solution is to readjust them all by hand in the master slides.
A workaround to solve the distortion:
- Make two copies of the template. Change only one of them to 4:3 layout.
- Enter the master slides in both templates. Learn how in the FAQs.
- Delete the distorted assets in the 4:3 one and copy there the right assets from the 16:9 template.
- Resize to fit the width and move as desired.