Larian has said you will be able to set the party size to six at game release. This is currently perfectly feasible for the game, but requires save game editing, so there is no reason for Larian not to deliver on this promise in the end.
There's some justifiable reasons they might not want to make this available during testing phases, though, mainly balance issues and similar.
Sources ? I only heard about mods, not official support.
It would be far more easy to balance the game with more companions. All those bonus actions and cheesy things are necessary because we don't have enough things to do during combats with only 4 characters.
You cannot on the same time ask for them to remove advantages from highground/backstab or shove as an action, or... without asking them to increase the party size. Combats would be even slower with a party of 4 and "a better implementation of the rules".