How do I make balls gather in a box w emitter?

I’m trying to create an animation, where a bunch of balls fall down in a box and slowly cover the camera.
I’ve used the particle system, made a group of 4 balls, set up a box w collision and turned on the emitter.

Even though I’m sending 10,000 balls through they never stack higher than this.
[ATTACH=CONFIG]373797[/ATTACH]

Any ideas as how to make this work?
These are my emitter settings



Help is much appreciated!

Thank you. Not what I was hoping but good to know.