Exchanging pieces is not a form of aggression at two-player chess, but things are different at four-player chess. By trading pieces with your left or right enemy, you make him vulnerable from attacks by your friend at the other side of the board.
What I don't like with the current system is that people offer trades, expecting for their offer to be rejected. Some become really irritated if you actually honor their offer. But if early exchanges were rewarded with extra points, then offering a trade will become a riskier thing to do. Especially if there have already occurred three captures earlier in the game!
The game as it stands now does not encourage early aggression. Exchanging queens early on is a terrible idea, since you get 9 points at the cost of becoming a target for further attacks, since you'll have trouble defending your base. But if you get 2*9 = 18 points instead of 9, then exchanging queens may become viable. So I suggest that the first 4 captures of the game give double the points, to reward the players willing to spill the first blood.