The party versatility thing is where I'm at for the 5-6 man team.
At 4 person, it's really important for each position to fill a role. D&D is still a game of specialized performers so while 5e allows more flex than a lot of past editions (there's a lot of different ways to get healing, and you could have a cleric as prime damage while a bard does prime healing depending on builds... for example) but you still need to consider these facts.
So, at 4, I'm going to have one support, one artillery, one melee, one utility. I can stretch that a little bit here and there. My planned party if, it stays at 4 is and assuming two characters are companions:
Karlach (Paladin or Barbarian?) for melee, Alfira (bard, support/artillery), Shadowheart (cleric, artillery/support), and a Monk, Rogue, or Ranger for my own character doing utility.
If Karlach and Alfira are not companion choices, I'm probably going Laezel (Melee), Wyll (artillery), Shadowheart (support), and Monk/Rogue/Ranger for my own character doing utility.
But stretching even to 5 would allow me to double up a roll and maybe explore one of those story lines where the character is a bit annoying, but the storyline is interesting and I'll have bear with them to see if they improve.
5 lets me build a balanced set of characters in the companions and then play whatever class/subclass I want just on pure concept for my own character without worries about leaving myself a vulnerability somewhere.
This would let me, say, have a run where I have a cleric or paladin of Selune romancing Shadowheart (because I'm a goof like that) or a second bard matching with a (please let her be a companion or at least a romanceable utility character that follows your camps even if she never joins the party for fights) Alfira. Without worrying, again, that doubling up would leave me vulnerable. (granted, a Light cleric is a very different beast than a Trickery cleric)