Make a white layer with a circle on it.
Delete the inside of the circle leaving the black line intact.
Use this layer as the front layer.
Place the layer with the animated rain behind this front layer.  :)

76% of original size (was 661x19) - Click to enlarge
YouTube Tutorials Here

Nice idea.  Thanks for trying to help me with this and for experimenting with this.

The way you did it, I can see that the white layer is masking the animation layer so that you only see a portion inside the shape. But it works if I am only using the shape with the animation inside and nothing else on stage.  
If I have anything else on the stage it gets covered by the white layer.

The idea is to have a Scene.  Then, on top of the that scene, to have a shape with an animation in it.  Also to be able to move the shape with the animation in it anywhere on the stage without covering everything else.

In Moho, the mask layers don't cover anything else and I can move it around on the stage.  The layer with the mask is independent from anything else and I don't need a white layer and a hole in the shape to do this  effect.   I am trying to recreate this in CA4. 

But you have the right idea.  For just the shape and the animation inside the shape only, I would use your technique if I didn’t need anything else on the stage.

Maybe I am still missing something or CA4 can't do this?

Thanks again!

76% of original size (was 661x19) - Click to enlarge

Hi vepop,
Here is what it looks like in Moho.  I can move the masked layer ( shape ) with the animation anywhere and it doesn't cover the background images.

Cheers!

Anything else on stage has to be on top of the white layer.
If the white layer is wall of a house with windows, delete the part of the windows to expose the rain layer.
Furnitures etc .. have to be in front of the wall layer. 
This is just an example.

76% of original size (was 661x19) - Click to enlarge
YouTube Tutorials Here

And it is a good example.
I'll play around with it some more.  It seems like a lot of work in CA4 just to accomplish this.  I guess I am spoiled with the way I can do it in Moho so quickly.  LOL
Thanks for your postings.  Have a great day!