And again one more time, just because you CAN get by with a 4 person party you can make the same argument for even smaller parties too until you are only playing the main character and your other party members if any are just uncontrollable NPCs. But would that game be BG? Not really. Would a 4 member party game be BG? Not really. Would a game that doesn't have a plethora of party members to choose from like the previous ones be BG? Not really. But would a game with a 6 character party and lots of different characters to choose from be BG? Well yes it would! Ding ding ding, we have a winner.
What made Baldur's Gate Baldur's Gate was strong characters and compelling story using D&D rules set in the Forgotten Realms. It wasn't the number of companions or that it ran on Windows 98 or the combat style any more than it was incredibly minimal character customization, or that it used pixelated sprites as character models. Latching on to these superficial aspects of a 20 year old game and insisting they are central to the Baldur's Gate experience is an exercise in futility.
Larian will be balancing combat for a certain size party in Baldur's Gate 3. They have chosen 4. That number isn't going to change based on peoples feelings of nostalgia for an irrelevant aspect of the previous 20 year old installment in the Baldur's Gate Series.