If the game allows a party of 6 (or 5) then you can still run with a party of 4 if you so choose. If the game only allows a party of 4 then you are stuffed.
A party of 6 (or 5) allows players who are new to D&D to have a standard/classic party of 4 and have room to experiment with one or two of the more 'exotic' classics such as, e.g. Warlock or with a multi-classed member.

I am firmly in the bigger party is better camp but I can live with a party of 4 when the companions are tolerable.

I'm not generally a fan of 'party banter' (especially where Larian's infantile 'humour' is involved) and the companions' back stories we have so far are more variations on a theme rather than an individual bio. They all have the entity in the background and most of them have a shit personality.
I recently replayed DA:Inquisition and the personalities of Varric, Leliana, Cassandra and VIvienne are streets ahead of anything Larian looks like producing in BG3. Minsc and Boo will not be featuring in my plays any more than they did in BG2.
On a side note I am interested in what rationale Larian have for bringing Jaheira and Minsc into BG3.

There are gaps in the logic of some of the companions: Asterion has been around for how long and is still level 1; Shadowheart has been through all sorts working for Shar and is still level 1; I'm not familiar enough with Gale and Wyll's stories but the same point could, I think, be raised. Only Lae'zel is 'new' to all this, or at least relatively new.

Then there is the poor ability stat spread that most of the companions have.
The most enjoyable run I've had with EA is with Gale modded as a cleric, Shadowheart as a paladin and everyone's stats re-jigged.

Like many of the others I am resigned to BG3 being a party of 4 and I currently view my best options with regards to companions as either a full custom party or mods.