Lowering a 4K TV's resolution to 720p or 1080p to match the input can improve the picture quality in some cases. This is because the TV will not need to upscale the image to fit the panel, which can result in a clearer and more detailed picture. However, it is important to note that lowering the resolution will also make the picture smaller, as the image will only fill a portion of the screen. Additionally, lowering the resolution too much can result in a loss of detail and clarity, so it is important to find the right balance.
Upscaling, on the other hand, is the process of taking a lower resolution image and increasing its size to fit the native resolution of the TV panel. This is necessary when feeding a lower resolution image into a higher resolution TV, as the TV needs to blow up the image to fit the screen. While upscaling can improve the picture quality to some extent, it is not always as effective as displaying the image in its native resolution. The quality of the upscaling depends on the capabilities of the TV and the quality of the original image.