Generally native resolution of the source for what it was coded for is the best because it is matched to the amount of pixels. If you put 1080p content on 4k, it will stretch the video across many pixels making it blurry. Some HDTVs do upconverting which helps improve the quality back to original source (1080p) and can slightly look better doing so. It depends on the capabilities of the source media and your TV.
One of the things you pay for in more expensive 4k TVs is how well the upscale to 4k. You can't really see the additional pixels in a 4k TV unless you are very very close to it. When you buy a better TV you will get fewer motion artifacts, more accurate color, deeper blacks, a better back light and HDR.