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Telly starts shipping its free ad-supported 55-inch TVs
(www.theverge.com)
This is a most excellent place for technology news and articles.
As already said you just need to buy something explicitly listed as a monitor.
One thing you'll find is that at 55" with decent specs you'll be paying a significantly higher price than you could find a 4k TV. That's because good monitors of that size are often super high end displays. E.g. Nvidia branded big format gaming displays.