The game was alright with four, the amount of side quests made up for the dungeons entirely. You see, Majora was a game of story and Aonuma wanted to bend the story around individual character strifes that would grow you accustomed to everyone in Termina. Through this strife you would feel an...
The problems with that are the creation of a new engine which would take at least 3 years if you count what happened to OOT's engine.
Then there would be the making of character models and such and the enviroments and they would have to get used to the technology.
Although if that did happen...
Majora had at least 5 temples when you involve the Moon and I find that enough, the thing with MM is that it had enough extra things that made up more than the dungeons.
Let's compare this game to MC. MC had 5 dungeons yet it felt much shorter than MM because of how fast you went to dungeons...