How do I change the format of the scene (4/3, 16/9)?

When in  a session mode you can change the format of a scene, by default it's set to 16/9 however you can choose 4/3 format.

On a Scene window choose option Format from Menu bar:

Please note Format need to be changed before adding any background or layered assets otherwise your background will be reorganized and all layered assets removed.