There are a few ways to force a 4K resolution on a 1080p monitor. One way is to use the NVIDIA Control Panel to change the Perform scaling on setting to GPU and then check the box that says Enable resolutions not supported by this display. Another way is to use the AMD Radeon Software to enable supersampling, which will upscale the resolution of the game to 4K and then downsample it to 1080p for display on the monitor. It's also possible to use third-party software or tools to add custom resolutions to the display settings and force the monitor to display a 4K resolution.
It's worth noting that while it's possible to force a 4K resolution on a 1080p monitor, the image quality may not be as good as it would be on a native 4K display. This is because the monitor is not designed to display 4K resolution and may have trouble handling the increased number of pixels. Additionally, the increased resolution may cause the monitor to display the image in a stretched or distorted manner.