The Collaborative International Dictionary of English v.0.48:

Four-o'clock \Four"-o'clock`\, n.
   1. (Bot.) A plant of the genus Mirabilis. There are about
      half a dozen species, natives of the warmer parts of
      America. The common four-o'clock is M. Jalapa. Its
      flowers are white, yellow, and red, and open toward
      sunset, or earlier in cloudy weather; hence the name. It
      is also called marvel of Peru, and afternoon lady.
      [1913 Webster]

   2. (Zool.) The friar bird; -- so called from its cry, which
      resembles these words.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

marvel of Peru \marvel of Peru\, marvel-of-Peru \marvel-of-Peru\
   n. (Bot.)
   A common garden plant (Mirabilis jalapa or Mirabilis
   uniflora) of North America having fragrant red or purple or
   yellow or white flowers that open in late afternoon; called
   also four-o'clock. See four-o'clock.

   Syn: common four-o'clock, marvel of Peru, Mirabilis jalapa,
        Mirabilis uniflora.
        [WordNet 1.5]