WiFi is half-duplex (send OR receive),
Ethernet is full-duplex (send AND receive simultaneously).
In terms of streaming video, you are mostly receiving data (for an AppleTV, it is very rare that you would ever be sending data), so having a full-duplex connection is irrelevant in this case.
A disadvantage to WiFi is that all devices connected to the same access Access Point are considered a collision domain (an area in which packet-collisions can happen). On an Ethernet Switch, each port is its own collision domain, therefore there are no packet collisions at all.
So, the more devices you have on WiFi, the higher the chances are for collisions to happen, which slows communication down for all WiFi devices.
Putting the AppleTV on an Ethernet cable clears up some of your WiFi Spectrum for the devices that need it the most (such as your smart phone).
In terms of data throughout on your 4th gen AppleTV, is a full-duplex 100mbps (Ethernet) better than a half-duplex 1Gbps (802.11ac)? In terms of streaming video, I’d say yes, but mainly because uncompressed 1080p video streams at 25mbps, which is only 1/4 of what the Ethernet port is fully capable of. You would get none of the disadvantages that WiFi brings (such as interference and packet collisions).
It is also worth mentioning that live OTA TV is not 1080p, it is either 720p or 1080i. DVD (480p) quality video is 10mbps. Therefore, live TV falls somewhere between 10mb to 25mbps (lets call it 18mbps).
With that said, 100mbps is plenty sufficient in this case. The benefits an Ethernet cable would bring is overall stability in that there are no packet collisions or interference, as well as better quality WiFi for the other devices.
4K video uncompressed is 100mbps, hence why the 4K AppleTV box gets a 1Gbps port.