It is generally agreed upon that 1080p content will look worse on a 4K monitor compared to a 1080p monitor. This is because the pixels in the 1080p content are being stretched to fit the 4K resolution, which results in a blurry and less sharp image. Additionally, most monitors and TVs add blur when upscaling content, which can make the image look even worse.
However, some newer monitors, such as the Eve Spectrum, have built-in pixel-perfect integer scaling, which can improve the quality of 1080p content on a 4K monitor. If your graphics card drivers also support integer scaling, you can double the width and height of the 1080p content to get a sharper image. On the other hand, some monitors downscale higher-resolution signals, which can result in a loss of quality when displaying 1080p content.