Why is it taking so long to download movies on my brand new 4K Apple TV.

1/ You can’t download (store locally) 4K movies from the iTunes Store. HD is the highest quality you can download.

2/ Apple recommends a minimum speed of 25 Mbps for 4K streaming alone.

3/ A strong Wi-Fi signal doesn’t say much about the actual bandwidth.

4/ Run the Speedtest app (also available on tvOS) to measure the actual bandwidth available to the Apple TV box.

Movies from itunes can't be downloaded onto Apple TV.

The signal shown on the device also doesn't account for issues kn the network. On top of checking your speed you would need a diagnostic report from istumbler, netstumbler or similar. This would show accurate signal strength along with noise and nearby networks.

Also make sure DNS is set to auto (settings-network)

Test on another network