[Confirmed: Pillars of Eternity 2: Deadfire]

Josh Sawyer confirmed it in a Tumblr ask (in German) http://jesawyer.tumblr.com/post/156419783766/hallo-josh-es-wird-gemunkelt-das-für-deadfire.

Google Translate:
"Hello Josh, it is rumored that for Deadfire the maximum number of possible companions was reduced to 4 (5 in PoE) and thus the maximum size of the group - including the guard* - was reduced to 5 (6 in PoE). Is there any truth to it, or is it only due to a write error or an unfortunate phrase on the part of the press? Thanks in advance.

 

We have not said anything about the number of the companions, but the maximum size of the group is 5 (including the guard*)."

 

*watcher
 

[Combat System? Potential Backer Question]

For what it's worth, Josh wasn't involved with Tyranny at all and hasn't stated any interest in borrowing from it. He's also definitely heard the criticisms about enemy density and "trash mobs" and will probably try to improve on that. They're also doing NPC schedules, weather and time-sensitive quests, so hopefully that makes the non-combat parts of the game much more interesting.

[Nonsensical Humanoid Behavior]

A very simple way to implement this would be to simply have certain kith encounters "call for help", so that if you attack one it will immediately aggro several other nearby kith. Perhaps this could be combined with a timer where if you kill them before the timer runs down, they don't call for help (allowing "stealthy" approaches and perhaps giving the Rogue an extra purpose).

 Josh Sawyer mentioned this in the latest livestream. This is actually in the game (and has happened to me several times), it apparently just didn't always work.