The Windows drivers for both NVIDIA and AMD GPUs have an option that makes the graphics card scale non-native screen resolutions up to the monitor's native resolution using the graphics card's scaling hardware (instead of the monitor's, if the monitor has any). It should be possible to run games at a non-native (lower) resolution and have the GPU scale the image up to native resolution (so it fills the screen) without a noticeable performance hit.
Also, large monitors have used tiled panels internally for a long time, it's just that the recent batches of 4k monitors are the first "mass market" monitors to expose the individual tiles directly to the computer's graphics hardware instead of using a scaler in the monitor to present the tiles to the computer's graphics hardware as what looks like a single monitor.
You can see the two tiles (1280x1600 each) in my Dell 3008WFP when the monitor's scaler manages to get the scanline ordering wrong on one of the tiles, like on this composite signal from an NES: