Albeit I love western CRPG games, as a hardcore fan of strategy games, I always found the fact that you can only use one stack of 6 soldiers at the same time pretty limited. I know in most RPGs you can recruit and switch party members and I appreciate that but I always felt constricted by the number of companions and classes you can use in the same run or at the same time. So a party of four will be a great downside for me.
I like to recruit all characters possible, keep all my recruits leveled up and equipped, switch party members to suit my needs whenever I want and also I like to see the dialogs of the characters in several places, watch different party interactions, etc.
That was not the case in DOS2 games. they decided to limit the number of your party members to 4, limited greatly the party interactions and they made switching difficult (but you can respec your characters at will). Luckily there are mods to fix that (I use mods to allow a party of six in DOS games and also double the number of enemies per encounter to keep the game interesting). The "6manparty" and "increased party" mods have 30K downloads only in steamworkshop so I think many people like it too.
A party of 5 will be a good middle ground, and improvement but still I prefer 6 or more. But always with the option to go solo for all those solo players over there.
Ed: Typos
I think it'll likely be either 4 or 5, because of the way challenge rating is calculated in 5e. It assumes that you have 4 players of the same level of the challgenge rating of the listed enemy.
But if they allow a companion of each class it'll total 12, so you'll probably be able to "store" the ones you are not using and switch your companions whenever you want.
