That was 3.
We've not got that far - only playing on Heroic on and off at the moment. My mate (bless her) isn't the greatest gamer - she destroys anyone else we play with in deathmatch but she's not got the head for sticking to one role, so although she's holds her own as a sniper and/or rambo, she always mixes up our roles - which is cause for much amusement when we both find ourselves one-man armying the same set piece.
I'd be interested to see any links you might have that suggest that - every video I've seen and every review I've read has focused on the MP game element.