Answer: The first thing that comes to mind are some hardy vines. A zone 4 climate will be fine for many blooming vines provided you have healthy soil and apply water as needed. Vines to consider would include the smaller types of clematis, perennial sweet peas (they tend to run rampant in all directions unless you are willing to tidy them frequently), 'Silver Lace' vine, hardy kiwis, or bittersweet, to name a few.
If yours is a low retaining wall and you'd like smaller plants, consider groundcovers such as creeping phlox (light shade, flowers in spring) and vinca minor (common name is periwinkle, part shade), or mat-forming perennials like evergreen candytuft and rock soapwort (Saponaria ocymoides)
You might also consider evergreen creeping juniper.
To help you in your planning and in the ongoing maintenance, you might wish to look at a couple of basic books. I
particularly like the Dummies series for straightforward, helpful information. "Gardening for Dummies" by Michael MacCaskey (ISBN 1-56884-644-4) will get you started and "Perennials for Dummies" by Marcia Tatroe(ISBN 0-7645-5030-6).
Good luck with your project!
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