Bedstraw Family, Madder Family: Square stems, often with tiny clingy hairs. Leaves whorled (The many leaves arise from the same spot on the stem, but on all sides of the stem.) The white flowers are usually very small, with the 4 petals shaped as a cross.
Bleeding Heart Family: The 4 petals are irregularly shaped and sized, either to look like hearts or spurred sacs. The leaves are parsley-like, but with rounded edges and a blue-green tinge.
Dogwood Family: 4 large white showy bracts which may be mistaken as petals. These white bracts surround a tiny yellow cluster of flowers. Leaves opposite. Dogwoods may be trees or small ground covers.
Evening-primrose Family: There are 4 of each of the petals, sepals, and stamens. The style (long projection from the central ovary has a 4-part cross (X) at its end, or is bulb-like.
Gentian Family: Herbaceous plants with petals and floral parts in 4s or 5s. The petals are united. The leaves are opposite on stout stems with entire margins.
Mustard Family: Urn-like flowers with 4 petals shaped as an cross or an X when viewed from above. The flowers have 6 stamens (4 above, 2 below).
Oleaster Family: Canada Buffalo-berry, Canadian Buffaloberry, Russet Buffaloberry, Soapberry, Soopolallie: Shepherdia canadensis (Synonyms: Elaeagnus canadensis, Lepargyrea canadensis) - Shrub.
Poppy Family: Four large, colorful, showy petals. Poppies have many stamens surrounding a swollen central ovary. The sepals fall off the flower as it opens.