If I am watching a 4K movie, will my data bandwidth from the internet be less or stay the same if I reduce my display type from 1080 to 720 in the Roku settings.
Yes, lower resolutions should reduce bandwidth. Exactly how much depends on the provider/stream. Watching a 4K movie in 1080, may have already cut it in half (or more) but going down to 720 should do even more.