Verify that the TV is configured to receive 4K content through HDMI. Some TVs need to be manually configured for high bandwidth HDMI throughput (e.g. “Enhanced format”).
Verify that the Apple TV is set to output content in 4K resolution. Settings﹥Video and Audio﹥Format﹥…
Verify that any equipment in between is HDMI 2.0 or later with HDCP 2.2 or later.
Verify that the HDMI cables are rated for 18 Gbps or better.
Most content purchased as HD would now be playable in 4K, if available as 4K. It would show the 4K badge for the title in the Library, even if equipment and settings were not in 4K.
A few titles have unfortunately seen a re-release in 4K as a separate Store item, not qualifying for the auto upgrade.
Content purchased in SD would stay in SD, and not play in higher quality.
Trailers may have a different quality than the full movie.
Downloaded content would max out as HD 1080p. 4K is streaming-only.