One thing to remember when planting a new bed is that as plants mature they get bigger so leave room for that, also leave enough room around the plants that you can reach everywhere to
weed them.
Next try to get plants with different bloom times. Peonies like part shade and bloom in May standing around 30 inches tall; but, they can take a long time to get adjusted to a new environment up to 8 years to bloom for some varieties. Primroses also like shade and bloom for a long time during the
growing season. Primroses look great in the front of the boarder since the blooms are very close to the ground.
And a slow spreading evergreen groundcover can give you that full look and 4 seasons of interest for your garden.
A small water feature can also add to the four seasons garden even if it is as small as a rock with a shallow bowl in it just enough to hold an inch of water for butterflies during the summer.
And lets not forget some kind of bench so the gardener has a place to rest after caring for the garden.
I would also look at some kind of lighting if the bed can be viewed from a good vantage point.