1 and 4 look pretty interesting. I didn't know XFrog had that snow adding possibility. So how did you make #4? Is that the Blender extra?
One way of adding snow to existing trees (I read) is to add another alpha mask, with, as you say, a slightly larger clumped snow area, covering most of the needles. If you add that as a UV image mask masking a surface shader, you would also be able to fractilize the amount I'd say. Preferable would be another mesh of course, a bit convex, but that would be another tree. Or you could double the branches, and replace one by that mesh, and choose wich one to use in TG.
This is what is said on the ST forum: However, if you look at the Scots Pine and the Colorado Blue Spruce, the snow was made by making a bumpy plane that sat on top of the leaf mesh (consisting of branch with needles). Then place the leaf map shape as the snows alpha so the snow will line up with the leaf map.
This technique helps prevent flatness on the leaf meshes and adds more reality because when you view the leaf mesh from above, you'll see the snow, and from below, you'll see the original leaf map with out snow. It gets a bit tricky though, because the snow texture and the leaf map texture have to be in the same image, so you'll have to line UV's up with mesh and combine the 2 textures in an image editing program.
