I don't care what kind of challenges you wanna overcome as developers with party of 4 or what kind of challenges you wanna put before players with party of 4. It doesn't change the fact that party of 4 is too small for an immersive DnD game.
It takes away a lot party setup possibilites and I fed up with going back to the camp leave some one and take Astarion whenever I need perception and sleight of hands rolls.
4 is a recommended party size for DND 3e, 3.5e, 5e, PF 1e, and 2e.
I don't know where you got idea that party should be bigger
(and any DM would say you they definitely prefer party of 4).
Less than 4 - not enough versatility to overcome challenges.
More than 4 - too little attention for each character, and combat becomes too slow.