Would be interested in where you read that if you find it again - below is a link to an article that says the optimal team size is 5 to 8 based on a Dunbar analysis.
3 in unstable as too often 1 person feels left out, 4 is an even number so risk of split vote.
I read that a long long time ago - but I'll have a think and see if I can recall where it was.
I'd agree that a larger team effectively mitigates risks around individuals not fitting in or decision-making being stymied, but in my own personal experience I've definitely preferred, due to a perception of maximising by own personal productivity, that 3 or 4 was best. That is, of course, completely subjective!
Technical decisions in a 4 person team should really be built on a consensus model rather than a democracy. Otherwise there's a chance of factionalism.
3 in unstable as too often 1 person feels left out, 4 is an even number so risk of split vote.
http://www.lifewithalacrity.com/2004/03/the_dunbar_numb.html