4b? Ouch. You could always try Datura.

Hello fellow 4-er!
There are lots of suitable white flowers that do well for us.
White daisies, esp. "Becky"
White Phlox, "David" "Miss lingard" ;
White Candytuft/iberis ;
Lilies both asiatic and oriental (and probably others) Casa Blanca does very well.
The ground cover "Snow in summer"
White swamp milkweed - asclepias incarnata "Ice Ballet"
White Daylilies - Joan Senior needs a bit if winter protection, well-drained soils and extra mulch but will make it. Other light colored daylilies will work and many stay open through the evening.
White roses - I have "Gourmet Popcorn" and love it! Also consider "Blanc Double De Courbert" or "Iceberg"I have David Austins' Heritage which is a pale enough pink to pass for white at night. Also in our zone, be sure to get own-root roses...they stand a better chance of getting through the winter.

For spring don't forget to plant white crocus,tulips, and daffodils.

You can grow white annuals like petunias, marigolds, sunflowers, moonflower, nicotiana and my fav the white version of the black eyed susan vine. Etc, Etc, Etc!!

Also don't forget variegated foliage plants! I have white nancy lamium as a ground cover, bishop weed and variegated hostas in the shade garden and I love my variegated red twig dogwood. It's by a window and the inside lights glow through the leaves for passersby, a very nice effect.

Lots of very hardy perennials have white varieties which will do very nicely! Good Luck!