Ghostofmacbeth is right, there is a rain material already in Bryce 3D (and I would assume Bryce 4 also). I've used it before and all I did was apply it to a cube, enlarged the cube to get the amount of space between the drops that I wanted, and moved it up in front of your feild of veiw so that you are looking through it to the pic. Here's a pic that I did a while back. I'm just posting it to give you an idea of what it looks like, not to display the pic, it's not that good in my opinion.