That's exactly it, and I really like your observation that the "limit of 4" doesn't apply in some cases. Before/after class, we have the exact experience you mentioned here where people will transiently go around talking to different groups (often 2 or 3 individuals, sometimes more where there are more listeners than speakers).
What's really interesting about your observation is how the rule-of-thumb breaks down when the conversation is limited to more confined topics or: If the individuals see each other with some regularity outside the group setting (no need to engage in extended conversation about who's doing what) or some of the conversations involve topics brought up in the course of the class.
What's really interesting about your observation is how the rule-of-thumb breaks down when the conversation is limited to more confined topics or: If the individuals see each other with some regularity outside the group setting (no need to engage in extended conversation about who's doing what) or some of the conversations involve topics brought up in the course of the class.