HDR and SDR describe different approaches to representing brightness and colour, while resolution describes image dimensions. HDR can preserve a wider range of highlights and shadows when the source and display support it. A 4K stream can be SDR, and an HDR stream need not be 4K.
Check the source and display
An HDR-capable television does not make every programme HDR. The app, decoder and output path must pass a compatible format, and the programme must have been prepared accordingly. A television switching into an HDR mode is useful evidence about the received signal, but it does not guarantee artistic quality.
Judge detail rather than maximum glare
Good HDR is not simply an aggressively bright whole picture. Look at sunlight, reflections and lamps alongside faces and dark backgrounds. A display’s tone mapping decides how the source is fitted to its capabilities, which is why the same HDR material can look different on two screens.
Use a sensible comparison
Compare the same title or a trusted sample in your normal room. Avoid forcing SDR into a permanent HDR output merely to keep a badge visible. If HDR appears washed out or unusually dim, inspect compatibility, picture mode and room lighting before assuming the source resolution or account access is wrong.