I think 5 members in the party would be better, one more person could make all the difference in a fight and it would also be nice to have an extra companion to interact with. I really just hope that more companions are available at some point down the road, I'm guessing making one companion for each class isn't really something they're going to do it would be fantastic if they did, because as things stand party composition is really limited. If the available companions are the exact same at full release, then we won't really be able to have a party based around which characters we like or which characters our character would have with them in terms of RP.
Let's say I make a stereotypical paladin who wants to smite everything he would consider evil. Welp, there goes Astarion, Shadowheart, and probably Wyll, since one is literally a vampire, one is an Evil Cleric, and the other made a pact with a demon. Now I don't even have a full party, and the only ones I have left are the Warrior Lae'zel (who I'd probably leave to do her own thing since she's just constantly a jerk anyway), and the Wizard Gale.
Now, I did find a side quest where I helped out a tiefling and it gave me the option to ask her to join my party, and she said maybe one day but she has business to take care of now. So maybe she'll be a possible companion at some point down the road, but there's still plenty of ways to put companions into the game as well. For instance, a druid companion could obviously be found in the druid's grove. Not every single companion has to have the same exact goal or some super mysterious backstory to make them a good companion.
A druid who decides to join my party to help out because they don't agree with Kagha's ways, and so they can kill us if we turn into a mindflayer? There's the motivation for coming with us. Doesn't need some super mysterious and amazing backstory like a wizard who might destroy the world or whatever if he dies, just a simple druid who wants to get away from her crazy leader and try to help keep people from turning into monsters, or kill them as a last resort. I'd take them with me, cause who wants to be a mindflayer?
Not every companion needs to be a playable character either, in fact having companions you can't play as might increase replay value. If I play through the full release game once with Wyll as my character, and then have Gale, Lae'zel, and Shadowheart for companions, then by the end I will have experienced every single character side story apart from Astarion. Even if I make a custom character, that would still leave only 2 companion side story's unexplored, and I'm not sure that finding out what happens to those 2 is convincing enough to sink another 60+ hours or however long into playing through again.
But if there are other companions that don't have some grand backstory that I prefer as companions over the other PC companions, then I'd take some of them along instead on that first playthrough. So say I play as Wyll, and fill my party with Shadowheart and 2 other NPC companions, that still leaves me with Gale, Lae'zel, and Astarion's side quests to discover, which might be enough of a change in content to warrant me another playthrough to see all 3 of their side quests. If I just play as Wyll and take 3 NPC companions along, I'm guaranteed another playthrough because I want to see at least 1 or 2 of the other companions side quests. All in all, it's easy enough to come up with ideas for NPC companions to join us, they don't all have to have an evil worm in their head or some insane backstory and side quest, they just need a motivation to join up with us and a personality so they can have some small chats with us and determine if they like or dislike our actions. Also, more romance choices :P