That would probably need 4mans in every other aspect of the game that there were 5mans. That seems to be both a reorganization nightmare for very little reason, or, it may be even very wrong to do it for other types of content (5man is already too small for dungeons). Also they can't have 4mans in raiding and 5mans or 6mans elsewhere because at the end of the day it's still a game and they want to keep a form of simplicity and consistency.