There are many varieties of cuphea you might try as a "filler". If you have the pot on a stand, you might also consider planting a "spiller" of cypress vine or cardinal climber (seeds are readily available at outlet stores and germination is easy--they self-seed like crazy!). I don't know what would work for a "thriller" in 4x4" cups...maybe a dwarf ornamental grass?

You might also try this question over in the Container Gardening forum...they'll be better at the planning side of things.

Here is a link that might be useful: Cuphea