|All photos copyright Marty Horowitz 8/20/2016
Plants in the Evening Primrose family (Onagraceae) generally have flowers with four separate petals. Hooker Evening Primrose (Oenothera elata ssp. hirsutissima) flowers are a fine example.
These plants like a little extra water. You'll see them blooming creekside through the end of the year (usually).