Choose quality on the service that plays the film
A title page identifies a production. The available video edition belongs to the viewing service you choose. Check that service's quality menu and requirements. A 4K display, a fast connection and a recent film are helpful ingredients, but they do not by themselves establish that a particular plan delivers that title in 4K.
Detail is not the only difference
720p and 1080p describe different picture dimensions. Compression, source material and motion also affect the result. Dark scenes can reveal compression more clearly than bright, still shots. If one edition looks poor, compare the service's actual options rather than treating every HD release as equivalent.
Let the picture settle
Adaptive streaming may begin at a lower quality and improve as the connection stabilizes. Repeated buffering calls for a different check: reduce competing traffic, try a steadier connection or temporarily lower the quality. Netflix's connection recommendations apply to its service; consult the equivalent guidance for the player you are using.
For a large television, also inspect the audio track, subtitles and any supported HDR setting. For a small phone, clear text and uninterrupted playback may be more valuable than the highest resolution. Use the Smart TV guide or data estimate for the decision that matters to your setup.