Flower:

Flowers are whorled around the stem at the top of the plant in a spike up to 12 inches long, blooming from the bottom up. Flowers are 1 to 2 inches across with 4 yellow diamond-shaped petals and 8 long yellow stamens surrounding a style with a cross-shaped stigma that typically rises well above the stamens. The 4 sepals behind the flower are narrowly lance shaped,  5/8 to 1 inch long, folded sharply back at flowering. usually in 2 pairs still connected along one edge (connivent). The calyx tube is ¾ to about 1½ inches long and resembles a flower stalk, attached between the base of the sepals and the ovary nestled in the leaf axil. The flowers bloom at night and on cloudy days. One plant may have several spikes, each at the end of branching stems.

Leaves and stems:

Leaves are narrow and often twisted with wavy edges, to 4 inches long and ¾ inch wide, covered in short hairs, mostly toothless, with little or no stalk. Attachment is alternate and the leaves are often crowded on the stem. First year plants have a basal rosette of stalked leaves that is up to 6 inches in diameter. Stems are mostly erect, unbranched or branched at the base, also covered in short hairs.

Notes:

Fourpoint Evening Primrose is on the Special Concern list for Minnesota. At Wild River State Park they've made a serious effort to repopulate the prairie areas with this plant and it seems to be doing quite well. Because of the shape of the petals, it is easy to distinguish from other evening primroses except for Cleland's Evening Primrose (Oenothera clelandii), which also has diamond shaped petals, but usually has smaller flowers and sepals, and its style not as long.