IF the game is done correctly, it should take into account that groups of 4 probably don't have all skills/approaches covered and thus offer more ways to achieve the parties goal. So in that case a party of just 4 would offer more replayability simply because you can just pick 80% of your party into the group taking the perfect solution in every case and instead just 50% forcing you to play in more variations to see it all. Basically you either prefer having variation in your 'mandatory core group + 2' or switching through the 3 companions next to your main - that's personal preference more than anything.
My feeling is for 5e 4 characters are the best feeling party composition when it comes to gameflow/gameplay. I understand the wish for 6, but especially if you have all your typical classes/roles it affects the gameplay into always having the right answer for the problem - which 5e doesn't really have at its core and it might take a bit of 'creative' problemsolving out of the game. I for example rather be not forced to have a primary spellcaster and/or cleric in the party like in other titles of this kind because I honestly hate both classes playstyles and don't want to even be bother with them if I can avoid them.
In other words - as long as I'm not forced to take a tank, a healer, a wizard and a rogue to be able to handle the game, I don't have any preferences if the party is 4 or 6 members. For me that is far more important for the quality of the game design than the size of the party.
On the other hand, as someone who has just so much time at hand and most likely won't play through the game more than twice in his life I would also prefer to see as much as possible of the companion stories at once... luckily I don't care about most of the NPCs so far, so maybe I will be able to see all those I care for even in a single playthrough XD
