First thing that comes to mind when reading your post is that blue star junipers look absolutely horrible in shade. They look as cute as a button when they're young and grown in full sun, but they tend to get pretty open looking in time, and they are just completely intolerant of shade.

As to whether it will hurt a tree to build up 4" of soil over the root system, it depends on the type of tree and the type of soil among other things. Some trees are more tolerant of root burying than others. Light, well-draining soil is less harmful than heavier soil. So, while you might get away with it, it's not a good idea and could damage or kill your tree.

My 2 cents is get some large growing hostas, liriope, ferns or fern leaved Bleeding heart. I would pick all of these plus some other shade plants. For no fuss the hostas can't be beat.

Blue stars like sun. I have a juniper hedge partially shaded by trees over the years and they're doing OK, but consider the spread on this type of juniper. Two feet away from a tree is just too close to allow it to attain a good form as it matures. I think I see your vision of soft blue/green living mulch around the trees, but if you can't put their root systems farther away and make your beds larger I'd go with something you can install and not tinker with the tree's environment so much. Most people make their under-tree beds too small anyway. You'[re always best to site shrubbery, or any plant for that matter, where it will fit at maturity without having to keep it pruned.