The biggest issue with this design is simply, how d you balance a 4-way map?
Let's throw out map design for now, since if they had this mode, it's obvious that map design won't be the premiere issue with it.
First and foremost, the only way you could set up a map balanced like this is to have a dual-axis symmetrical map. This means, squares and circles. Right off the bat, you're limiting map design. Three way would require circles and triangles, 2 way squares circles curved lines etc.
Now, you could do a four way "spiral" type map or "tentacle" style, whereby you can have a central meeting ground and all the arms where the bases are are sorta artistically positioned, but here's the problem. You have one army going to the center, that then has to split three ways.
So, first, you have a massive deadlock in the center, but then once you push through, you have severely reduced forces trying to assault up one arm of the map with a now-limited army trying to push against an army designed for 3 teams.
Okay, that idea won't work. So now you'd do a more scattered one army per base split with 3 paths. Now you have 4 teams in, let's say a square. Now you have 3 paths valid for each army to assault from, but only 1 path for each army to be assaulted from by any individual army (barring any previous armies being destroyed). So, now, you have one to your two corners on the right and left, and then one path heading towards the opposite corner.
Here you run into another issue. You have the opposing corners trying to fight each other. What will inevitably happen is these 2 opposing forces will now clash in an eternal deadlock, limiting exactly who you can hit to just your two sides.
This issue can be solved by an overpass/tunnel system, but I don't even think the supreme commander engine is capable of doing this (though the architecture leads me to believe it may be possible, we don't honestly know at this time)
When you introduce more armies, you have to redesign the maps completely around that particular lineup. You can't just convert a pre-existing map to more armies expecting it to be balanced because these types of maps also have moving armies in them attacking a specific goal.
I honestly don't think it's possible, if only because it severely limits map options. And, the more teams you add, the more likely you are to run into issues with balancing them against each other, pathing their armies from one to another, etc.
However, for all I know, none of these will be an issue and I'm just doom and glooming for no reason. It'd be fun to see team games with 3 or 4 teams, or even a FFA style game (though that'd be a nightmare to set up beyond 1v1) but, it is beta for a reason, no reason to not suggest something like this, I s'pose.