Regarding Sakura Con, I heard from a staffer that they don't do four days because it would make admission a lot more expensive.
Although I like the idea of conventions being four days, I think it works better for some than for others.
Anime Expo and Fanime have done well as 4-day events, but for other cons, it's better being three.
I remember people having mixed views when Kumoricon announced that it would go four days and I felt that it wasn't organized as well in its second year under that format.
I like anime conventions better when there are a lot of fan panels, but a good number of guest panels as well, and Kumoricon didn't deliver with the latter last year.
Before that, they were better with it, but last year, I felt it was a lot more lacking, both with number and variety.
I heard that there would be scheduling changes for this year, so hopefully there'll be a better number of guest panels this time while being more diverse as well.