They confirmed it will be 4. Also you will be able to rebuild the companion from scracht as in DOS2. Lets think I want Minsc on my team but I dont want a ranger cause im already a ranger, so I change Minsc and he is a Wizard now.
Thats sucks for me. I want to have tons of companions and explore different options to play again and again. If that evil cleric doesnt fits in you party maybe u should change the whole party, cause you want him/her. But probably they will follow the same way as DOS2. They had 4 companions in DOS1, 6 in DOS2. So I think there will not be 18 as in Baldurs Gate, all with great stories.