I too support some minisodes or an hour long special to tie up the loose ends of the series. But a fourth season would just be endless filler since there wouldnt be enough story to justify 20 more episodes if its used as the means to answer the rest of the questions (since there are like 5 questions that need answered anyway)
Besides, even if they cheapen the show by doing a spin off or creating another story or villian for the gAang to emmerse themselves in or go up against, that wont be enough for people and there will be petitions to create a 5th season to answer the questions left by the 4th season (if any). It will never end and Avatar will become the subject of mockery by the general public and we would no longer be fans of one of the greatest shows ever made, we would simply be a group of people sticking around for what was once a riveting piece of storytelling.
I respect and love Avatar too much to allow that to happen. Hell i think even the movies (which im obviously against) would cheapen the show and put the nail in the coffin for Avatar in the eyes of the general public who could've become Avatar fan's