Oh, I envy you the Mertensia!

Viola hederacea probably needs to be taken in for the winter unless you can guarantee frost protection. Same for the Cyclamen persica(?). The little hardy Cyclamen will probably last better, but they do like some sun.

Are you planning to use the coral berry as your source of interest over three seasons? Does it matter if the area is peaceful over winter? Perhaps you have garden ornaments that come into their own at this season, or interesting rocks/driftwood.

The Ajuga and Convallaria would go together but both need a bit of supervision after a while. I find they both do better florally with morning sun, otherwise there's more leaf.

Lantana, and the Begonia and the Impatiens will cover the summer - especially if you have a warm place to hold them over the winter. Athyrium nipponicum - especially one of the cultivars- could give summer interest and be dormant for winter. Hardy to zone 4. Fuchsia is also a possible with Impatiens, but do take the time to pick harmonious colours...

Hosta plantaginea is winter hardy in your zone and has scented flowers as well as dramatic leaves. Also nice with Impatiens/busy lizzie.

Having come back to earth: you could use the plants that are hardy to your zone in the ground and add interest by using the more tender plants in pots that can be taken away when they're not at their most interesting.

Or go primarily for foliage and form, with garden ornamentation - possibly a small water feature. Mossy rocks, ferns, Hosta, and little spring surprises. I also remind about Epimedium...diphyllum, x rubrum, x versicolor, etc.