In Firefox and Chrome, F11 goes into full-screen borderless window mode on the current monitor (not the one with the most pixels - I've never seen that behaviour; maybe a coincidence because the monitor with the most pixels is usually chosen by the user to be the 'main' monitor?).
IMO, it would make sense for Shift+F11 to mimic that behaviour (full-screen borderless window on the current monitor), Shift+F12 for full-screen borderless window across all monitors, and keep Shift+F4 for exclusive full-screen toggle on the main monitor.
FWIW, full-screen toggles are very inconsistent across windows applications - common ones are F10/F11 (and combinations with modifiers), Alt+Enter, <modifier>+Space and I'm pretty sure I've seen something other than LFS use something and F4 as well.
As an aside, I currently have a two monitor setup, but with different sizes/resolutions. I wonder what the "all monitors" mode would do on this kind of config.