The dynamic when I ask them to group up in fours or fives is completely different, as well when I have them work in pairs. Three is better, at least for the types of things I am asking of them. You might find Elizabeth Cohen's book, Designing Groupwork, to be of some use. I think she recommends a particular number for small groups, in that book, but can't recall what it is, offhand. Strategies for the Heterogeneous Classroom.
Students often enter courses with expectations of working alone.
The world is, however, not like this and we need to aid them in working together. This is difficult within a culture which praises the standalone genius.