Dames Rocket (Hesperis matronalis) 2016-5-31 White Clay Creek SP, DE

Can be confused with Phlox, but Phlox has 5 petals - not four, and opposite leaves - not alternate. Dame's Rocket is introduced and can be locally invasive.

There are differing descriptions of leaf shape depending on various sources, some saying "round" and some "lanceolate". However, florafinder.com seems to cover both bases with toothed leaves that are variously egg-shaped (ovate) or thinner and longer (lanceolate).
http://www.florafinder.com/Species/Hesperis_matronalis.php

Dame's Rocket has basal leaves that are withered by the time the plant flowers. As the leaves go up the stem they get smaller.
photo and description of sepals here:
https://www.minnesotawildflowers.info/flower/dames-rocket