I prefer three tune tandas. I think it encourages dancers to dance with more partners, including some partners they may not normally dance with. I think this is good for promoting greater mixing in a group.
I also prefer 3 tunes for regular tango music. One song to get acclimated to the new partner, and two to enjoy (hopefully enjoy).
Four songs gets long when the partner is not someone I really love dancing with. (and I think it makes my potential partners pickier too) Typically at least half my evening will be with people who I do NOT
really love dancing with. At least one or two tandas will be with people I'd rather not dance with at all, but can't refuse without awkwardness or unpleasantness. Hopefully at least 3 tandas will be with some of my favorite leaders. The rest of the night is with leaders I like ok, but won't go home sad if I miss that night.
Even though my list of leaders that I really like is getting longer because I am purposefully trying to find reasons to enjoy more leaders even if they aren't the cream of the crop as
dancers (and the overall quality of leaders here is improving steadily too) the above stats mean that I will spend a greater part of my evening dancing with folks who I might enjoy but don't get all
that excited about. So longer tandas would not be as nice.
I'll add that the leaders that are my favorites are favorites of quite a few other follows as well, and they are also people who try hard to dance with many followers over the course of the evening. So yeah, if I'm dancing with a favorite leader, I'd LOVE for the tanda to last a good 20 minutes (or all night!) but I also realize that there are other equally skilled followers sitting out because of the gender ratio and that next week I might be the one who didn't get to dance with a favorite partner if the tandas are long (meaning fewer of them).
Around here, the general pattern is to play more traditional music early on and segue to alternative as the night goes on. So, often as the individual pieces are getting longer, the crowd is also thinning and it's the die-hards left on the floor. This may explain why it never seems to be an issue that the tandas got longer at the end. There are fewer people left, and most of them are dancing, not sitting out.
Usually also, the people who stay later are also the ones who are more experienced. Beginners don't hang out for 5 hours. People who have sat for several tandas in a row by 11:00-12:00 end up leaving. Most people have a sense of when the best part of their evening has past and they pack it in.