This is a video game, and its about fun. Which means, its all about feeling in the first places, there is no logics to what people ''feel'' is fun, though they can try to explain it. In my opinion most of what we know of game design is empirical. like ''that game did it and its worked, let do it and tunes it a little differently''.
It is wrong. None of us knows enough about game design to properly argue about the difference having 4 or 6 companions makes in this type of game, in design and the way player engages with content, but game design isn't magic. You put things in front of the player and they respond to it in predictable ways. There is a lot of research and understand in why developers do things certain way - things we don't quite comprehend or think about when playing the game, but it doesn't mean there isn't logic, or "science" in there. Larian has 4 companions, not because it doesn't matter, but because it fits better to what they are trying to achieve.
And yes, different people find different things fun. But that game that is claiming to be Baldur's Gate3 isn't "fun" for (some/many?) fans of Baldur's Gate1&2 is a problem, even if those fans might find it difficult to articulate how individual changes change their experience with the title. Might not be problem for Larian, if they have large enough audience anyway, but it is something worth talking about.