And with “What about the poor devs who would have to rebalance” (a game that in the current state was never final to begin with and with most encounters still in a tuning phase) we have just hit the bingo card of the recurring arguments.
Because apparently Larian can implement fifteen difficulty settings in each one of their games, including one for vegetables who expect just to breathe as the game solves itself, but asking to support a larger party in lines with the standard of the saga is peak entitlement.
Honestly, would they even need to rebalance act1a at all? It'd make the act a bit easier sure, but this act is the introduction and therefore the difficulty increase would be more gradual I'd imagine. Act 2 and 3 would possibly need rebalancing but those acts would be expected to be harder so difficulty increases would work.
And like I imagine the game wouldn't be much harder with four party members instead of six. Since the difficulty range would likely be intended for 3 to 6 characters. Especially if they stay closer to base 5e in monster structure with their homebrewing in line with base 5e difficulty.