To prevent your TV from automatically changing from 4K resolution to 1080p, you will need to adjust the settings on your TV. The specific steps will depend on the make and model of your TV, but generally, you can find the resolution settings under the Display or Picture options in the TV's settings menu. Look for an option that says Resolution, Output Resolution, or Video Output Signal, and select the option to set the resolution to 4K. Additionally, you may need to disable any upscaling features on your TV, as these can cause the TV to automatically lower the resolution to 1080p. To do this, look for an option called Upscaling, Resolution Enhancer, or something similar, and turn it off.
It's important to note that not all sources of video content will be available in 4K resolution, so your TV may automatically switch to 1080p when playing content that isn't available in 4K. In this case, the best way to ensure that your TV stays in 4K resolution is to only play content that is available in 4K. Additionally, if you are using your TV as a monitor for a computer or other device, make sure that the device is capable of outputting video in 4K resolution. If the device is only capable of outputting in 1080p, your TV will automatically switch to 1080p to match the input signal.