I've played a few games that were sorta team games.
The only rule changes to the 4 player games were:
1: You don't ever fight your allies. If you both share a hex you don't engage in combat.
2: Each team needs to rack up double the number of points that would be required in a non-team game.
Not quite correct. You play to a combined 20 points (rather than 12 points) and you can give one card a turn to your partner.
Universal Head's Player Aid wrote:
Set up the game for a 4-player game and split into two 2-player teams, with partners sitting across from each other.
The first team to reach a combined total of 20 VP wins.
At the end of a player’s turn, his partner may pass any one card to him face down.
If a player moves into a hex with any number of his partner’s units, there is no battle, but the hex is still considered contested (eg. neither player draws rubium from a mine there). Units may move through a hex containing partner’s units as long as there are no opponent’s units there.
All discussion must be public, and a card shown to a partner must be shown to all players (not including the card passed at the end of a turn).
If a player is eliminated, his partner may continue to play for victory, still counting their combined VP total.