Playing a two-player game on L+U difficulty at 160% usually lasts from 3 to 6 weeks of game time (rarely longer). I think that's not very long. If 4 people play, I don't think the game time will increase much. Although it will affect development, as resources will now be divided among 4 players. You can simply test it: generate an M map for four players and try playing against the computer. And you'll understand how cramped it will be. I guarantee that encounters will happen as early as the first week.
In that time, a player might simply give up, realizing that their opponent has developed well, and they themselves have almost no chance... The actual clash can only occur if the starting locations are close together on such a large map... IMO, this is the downside of randomness.
When playing with four players, the time will definitely increase because the "win or lose" tactic is no longer appropriate, and everyone will be more cautious.
And regarding testing an M map with computers, you're exaggerating... First, I was talking about M+Dungeon, and second, what's there to test? Take any map from the "Seasons" era and play with four players... there are at least 4 castles on almost all of them... these are quite playable maps, and that's despite the fact that almost all of them are without a dungeon!!!

