That is an attractive shed. The direction the doors face will have important consequences for light. If you know that, do tell, though sun and shade options can both be suggested.
You want the plant to be four feet tall, including a reasonable container height, so perhaps a two and a half foot plant. Evergreen.
You mention shrubs (and trees), but you may wish to consider perennials as well. Low maintenance is a relative term. For containers, regular irrigation, and occasional fertilization, grooming and shaping would be expected.
If the shed faces South perhaps something like Lavender (Lavandula spp. & cvs.) or a low Rose (Rosa cvs.).
if the shed faces North perhaps an evergreen Azalea (Rhododendron, various evergreen hybrids) or Japanese Daphne (Daphne odora).
For East and West exposure with a half day of sun you might try a number of sun or shade options.
Again, for both sun and shade, consider a mixed planting of perennial and annuals in a large container. This often give the most interest and show through the seasons. Also, if you do want a taller shrub or less maintenance, consider an in-ground planting in that location, such as a Camellia or Hydrangea.
Finally, to state the obvious, I have only touched on a few possibilities, there are many others.
