In 2023, teams could add a third QB to the game day active list without taking up a spot. But that QB had to be on the 53 man roster.

But in 2024 the rule was changed to allow the third QB to be on the practice squad (not the 53 man roster) and still be activated on game day. Normally you can only do that so many times before having to add the to the 53... But that's no longer the case for the third QB.

Given that rule change, teams are now strongly disincentivized from using a roster spot on a third QB. Which is why Hall and Slovis fall where they do.

QB4 will be straight cut, QB3 likely ends up on the practice squad. The only reason you'd roster a third QB is if you want to protect them from another team getting them...which probably isn't the highest priority if they're your #3 QB