That's actually a misconception, though one that was often propagated so it stuck, unfortunately.
The Steam Deck will dynamically allocate video memory - way below 4GBs, or even larger than 4GBs, regardless of what you set as the UMA framebuffer setting. And it will change this allocation in real time, during each frame, as it monitors memory pressure.
The UMA buffer will indeed give a default "the GPU would like to report that 4GBs of memory are mapped to itself when the driver loads" but the CPU can (and will!) immediately ignore that as soon as needed.
Don't believe me? If you got 15 minutes to do a little experiment, try setting it at 256MB. 256MB for a device running modern games? No way! It won't get past the loading screen, right? Well, it will, and the performance will be quite literally identical to setting it at 4GB.
Addendum: some games give warnings of "Your system do not meet the minimum requirements of VRAM" when running on systems with APUs, in that particular case, setting a large UMA buffer will probably work to bypass the warning... But again, performance is similar.