Sounds like nVidia wants you to set the game at a higher resolution and then downscale it to fit your actual monitor resolution. I'm not sure exactly why, but nVidia is a bigger proponent of downscaling than AMD, even though both GPU manufacturers are proponents of features such as DLSS/FSR to upscale and generate additional frames to bolster gameplay.
Any GPU will work significantly harder while pushing 4K output to a display than it would 1080 or 1440 resolutions. Additionally, most 4K displays are not capable of the higher refresh rates that 1080/1440 displays can handle. This would mean that playing a game at 4K resolution would not only tax the GPU harder, but also sacrifice framerates in the name of resolution. When downsampling is enabled, it can improve the graphical quality on lower-resolution displays without the framerate hits that native 4K brings with it.
Unless the back of the case is the flimsiest of metals, the GPU is already supported on the back end due to it being secured to the back of the case, and the proximity of the PCIE slot to the back of the case. While many motherboards now reinforce the PCIE slots used by the GPUs, some of the longest and heaviest cards can still wreck a motherboard if left to sag. Some motherboards and even PC cases come with a bracket that just holds up the far end of the GPU because the back is already supported sufficiently.