I just tried this again to verify the Roku-4 sequence, and you could try waiting a little longer before the last up-ok. Here is the sequence again.
Home home home < gets you to home screen row 1
UP < gets you to Settings
OK < go into settings
Down Down Down < get to Display Type
OK < go into Display Type
Down < goes to 720P TV
OK < selects 720P TV
wait 5 seconds
OK < go to 720P
wait 10 seconds ( or longer since you should see the correct display now)
UP < goes to "OK 720P looks good"
OK < selects "OK 720P looks good"
And now you should be set to 720P