The four white "petals" aren't petals at all, but rather bracts, which are modified leaves. The actual flowers in the picture are those tiny greenish-yellow objects in the center of he thing people usually call dogwood flowers, but which is actually a flowering head.
The picture at the left shows a cross-section of the above flower head. Now you can better see the actual individual greenish-yellow flowers. If you don't know about stamens and sepals, before you read on you may want to check out our Standard Blossom Page.
Notice in the picture that each flower has 4 stamens, four petals and four sepals. The blossom in the center of the cluster opened first and already has lost its petals and stamens, leaving only the slender style arising from inside the cuplike calyx, which rises atop the ovary. In late summer and fall, these ovaries will mature into pretty red fruits.
The picture at the right shows just one dogwood flower, with the most obvious parts labeled. The whole thing, from the bottom of the ovary to the top of the stamens is only about ¼-inch (7 mm) tall.