Hi Seeker, welcome to the boards.
4-players would be nice but would require a lot of work to update. First, the mod is 320x240 and you'd have to change it to be 480x640 so its not too crowded. That would require editing all of the backgrounds to make them taller, and changing all the enemy spawns.
Then, you'd have to add in extra enemy spawns that are conditional (they only spawn if there's 3 or 4 players) so the game's not too easy.
The sheer amount of work involved, and the difficulty in finding the free time to do this means its not very likely in the near future. However, if someone else wanted to edit the game to add something like 4 players, they certainly have my blessing to do so!