Who is to say that 4 main dungeons all? It could be that there are 4 other dungeons prior to the giant mechas required to complete. Like, a dungeon key. Having to beat the Water dungeon to get the Desert key. Having to beat a fire temple to get the Forest dungeon key. Could add 4 more dungeons that way. Something that makes it so you can't just walk up to the mecha camel and go inside. Maybe...