While the support itself doesn’t necessarily mean OS-level toggle like the Steam Deck has right (which I believe is still at FSR 1), it still could be a nice touch for the titles that choose to implement FSR 4.
Maybe the community adds the OS-level support unofficially, somewhat similarly to Lossless Scaling right now (although perhaps for free). This is just my uneducated guesswork, please see this comment for proper expectations:
Proton-CachyOS also can upgrade FSR3 games to FSR4 on RX 6000 and 7000 series of GPUs, and it works outside of the Steam Deck platform.
Optiscaler (with Windows games) also works if you need further tweaks or run into compatibility issues with the Proton level support.
That’s partially true, unfortunately some games do not work with the FSR4 upgrade via OptiScaler, and on top of that there’s a plausible risk of getting banned in games with anticheat