monitors are usually a different aspect ratio to a TV
What? Aren't like 90% of monitors and 99% of TVs 16:9? There are a few monitors that are 16:10, some extremely rare 5:4 and 4:3 and then there are the ultrawide monitors which are obviously a different aspect ratio but saying that monitors are "usually" a different aspect ratio is factually incorrect. If you're deciding between a 4K TV and 4K monitor, then there's no danger of accidentally buying something of different format.