First let me say that I really like this concept/idea/playthrough

Second, if you didn't read it through already I suggest you read the Coldest War let's play by bgreman on the somethingawful forums. Excellent example, though there the players control just one nation, and the GM controls another.
That said I see a lot of problems with having 4 nations. Like, a LOT of problems.
Be it trading, tech trading, military cooperations, alliances or the like, there are too many possibilities of one or two nations being knocked off really quickly, thus halving the people who can play or so. Perhaps not killed, but effectively neutered as far as the possibility to expand goes.
Two nations cooperating could easily and quickly dispatch the other 2, be it through tech, wealth, or military assault. There is not much the other 2 nations could do, especially if they do not see it coming. Wars in aurora can last just a few hours, so the other 2 nations could realistically not defend themselves. And once you've lost all your ships/ installations away from earth, starting again from scratch is basically impossible. Also you risk a few charismatic/sneaky characters influencing the game too much with that, thus ruining the fun for others.
I am also against any "arbitrary" ban on trading or such, because it does not make sense.
A few possible way to deal with that:
- have just 2 players-controlled nations. If you want, you can control a third. This way you do not risk the players knocking themselves out too quickly, because they are in direct competition and have no reason to trust each other.
- Have more than 2 nations available, but give them "limitations". For example you could have a "peaceful, democratic" nation where the public opinion is very much against war unless first attacked, and another "xenophobic, isolationist" nation which does not trade/cooperate except in some rare cases. Basically, every nation has some situations which limit the players in some way thus preventing the race-to-win.
- Implement via RP a "spy system", which ensures a certain probability that player deals become public and thus become known to the other nations. This only works of course if you enforce all deals to pass through you. But it does not really works if the players decide to help each other outside the accepted channels, like via PM.
Regardless, having more than 2 players controlled nations could become ugly for a number of reasons in my opinion. I do heartily approve of the game concept though
Edit:
A couple of other reasons to keep the number of player controlled nations down:
- it would be hard to keep everything going (orders, queues, exploraion and the like) with ONE player controlled nation (because the players must agree upon the orders and such), let alone 4. I mean, it's possible but.... how much time you want to dedicate to this? :p
- in this game, like ALL other community games I've ever saw online, there WILL be defections. People stopping because of real life/boredom/no internet/whatever. With 4 player controlled nations, just a couple of defections could end up in a zombie nation, devoid of all important command positions. I think keeping the nation number low would help in ensuring a healthy amount of players per nation.
So, to sum it up, I think doing, say, 2 player controlled nations would be more realistic and more likely to succeed in the long run.