Hi
I know that probably someone will do a Mod about it when game release

But I think that D&D party if more complete with 5 People in it (or 6 like the Old D&D games) then 4.

4 is more for Games like Dragon Age or Divinity Original Sins
Games where you either have Fewer Classes and so you have a "Duplicate" (IE Dragon Age) or No Class and everyone can branch out (IE D:OS)

But in D&D its very limited if you want to cover all the bases
Especially if (like me) you only care about Scripted Written Companions

A typical Group will need a Cleric/Bard/Druid as a Healer, a Sorcerer/Wizard for AoE Blaster, a Rogue/Bard/Ranger for Pick-Locks and Skill Monkey (Rangers are here only because of D:OS take on Rangers, in Vanilla 5e they wouldn't be)
and a Warrior/Barabarian/Druid/Paladin for Frontline Fighter

1. you will notice I didn't say Warlock, because while good Damage Dealers they do not supply any of the Above
2. You feel semi Obligated to take one of each.
3. there is only 1 Companion for each "slot" of those (plus Warlock)

This feels very restricted to me

I know its not really a big issue and that (like mention above) Mods will allow this for sure
but I think games like this would be so much more fun and flexible if you had a 5 man party like Kingsmaker or Pillers 2