I'm pretty sure 'player characters' is referring to companions and other people; it wouldn't make sense to give a character a different bonus depending on whether or not one of the characters is controlled by another person (ie 1 party of 2 in single player vs 2 parties of 1 in co-op).
Someone tested it, the bonuses scaled with PLAYER characters, not companions. A party of 4 gave the same bonuses, but a party of 2 where both were real people gave less benefit than the full four man group where 3 were NPCs.
Which is admittedly how I prefer it, so that you can have a main character who feels special without being stuck without party interaction.
