I think it can be expected that at least the machine that renders the animation for the winner will be able to take advantage of full 4 GB. As each shot in the storyboard is not wider than 640px, the renders will not be so tough on memory as if your render was 1680 or 1920 wide.
So bottom line, I think you will be able to cram a whole lot of objects into the scene. But you are right in that this is not really easy this time, depending on your animation path: if your camera floats a long way close to the ground, you may not be able to cover it all with grasses, for example, so you need to get creative, and make intelligent choices with your scene and storyboard. And going for a more barren landscape is one option, for sure.