What does 4:5 mean in the Photo Editor?

It is the shape of the frame your picture is being fitted into — four units wide by five tall, the proportions of that slot in the cover design. You cannot change the shape, because it comes from the Cover template you chose, but you can decide which part of your photo sits inside it: drag the picture, use the zoom slider, then press Apply. A different Cover template gives you differently shaped frames.

This applies to cover pictures. The photographs inside your book are placed and sized automatically and cannot be edited, cropped or filtered individually — to change what appears there, use Edit content.

Where to find it: Photo Editor, bottom bar, beside the flip buttons.