The differences between the two devices can be found here; Apple TV - Compare Models - Apple.
You can see the Apple TV 4K has a better processor, Gigabit Ethernet as well as better Bluetooth and simultanious AC Wifi Support. Visually, the two devices are identical, other than the 4K version doesn't have a USB C port on the back for servicing.
I own the Apple TV (4th Generation) and within this, you can tell it to output right the way down to 480p so theoretically, the 4K version should be able to be set to 1080p. Again, I haven't tested the 4K too much to notice any major differences, but the upgraded chip is mainly there to run the ouput at 4K.
If you intend to invest and stick with the Apple TV for a while, then I would suggest the 4K as the newer processor and 4K output will come in handy if you ever deciede to upgrade your TV.
Hope this helped.